HEATING AND IRRIGATION - Greenhouse Vegetable Gardening (2015)

Greenhouse Vegetable Gardening (2015)



As a landmass, Sweden is very elongated; its climate shifts dramatically from the north to the south, and it also seems to change over time. The greenhouse can minimize these variations to some degree, but it can't eradicate them entirely.


Polycarbonate is recommended for use in the north of Sweden, as it insulates better than glass.

The old Swedish saying, “If you turn Sweden around, geographically you’ll end up in Rome,” is worth remembering when choosing a greenhouse. The difference in climate between Malmö, in Skåne, the southernmost province of Sweden, and Rome in Italy, is dramatic; the same holds true for when you compare the climate in Malmö to the climate in Kiruna, in northern Sweden. This means that greenhouses in the south will have radically different requirements from those in northern areas, so it’s a good idea to seek advice from locals before you set about shopping or building your structure.

Commercial growers have completely different needs from us hobby gardeners, so it would be unreasonable to compare our wants to their professional-grade requirements. The nursery pro needs to be able to control heat, irrigation, light, and ventilation with a great deal of precision, and they usually raise one type of crop at a time, depending on the season. We hobby growers, on the other hand, often keep an eclectic mix of plants, all with different needs; wishing to have temperatures calibrated to each individual specimen is unreasonable. Commercial growers build greenhouses adapted to the crops they’re raising, while we buy house kits chosen according to the size of our yards as well as for looks. The pros have computerized heat and irrigation; we can do little more than hope for a warm spring.

In addition to its surrounding climate, the greenhouse’s intended purpose is important. Unfortunately, climate will be the primary influence on your choice of plants. A simple unheated greenhouse can be used almost throughout the year in the south of Sweden, whereas in the north, a realistic timeframe spans only a few months. Further, it’s important to distinguish between actively cultivating crops, and overwintering only. To cultivate in wintertime you’ll need to warm up the ground with buried heating coils, a heating mat, or hot water pipes. If you only plan to overwinter plants, you’ll only need to supply above-ground heating.

Greenhouses in the south of Sweden

In the south of Sweden, a greenhouse is a simple and effective way to shorten a long winter. Its typically mild climate means that the greenhouse will work very well, without a lot of extra effort, for overwintering delicate plants. With the assistance of an electronic frost monitor and bubble wrap, you can keep the greenhouse frost-free without incurring a lot of added expense, thus making overwintering fuchsia, geraniums, bulbs and rhizomes, angel’s trumpet, heliotrope and potato vine entirely feasible. Seed starting of both summer flowers and vegetables, as well as the propagation of houseplants and other plants, will be both rewarding and quite simple to do. You’ll have to make sure to ventilate the greenhouse during the day to lower the temperature to stop the plants from developing too quickly, and to protect them from potential cold snaps in the winter. Occasionally, even the south of Sweden goes through a particularly cold period—usually towards the end of January—in which case you’ll need to run a fan heater during the cold spell. The snap is usually short enough, though, not to warrant the costly purchase of a permanent heating system.

Greenhouses in central Sweden

In central Sweden, a greenhouse is a great boon since it allows you to prolong the growing season—spring arrives earlier and fall lasts longer before winter sets in—if you use a fan heater to keep temperatures reliably above freezing. It’s also a good idea to have some type of insulating cover to keep out the chill, and a combination shade and thermal fabric is the way to go.

Sadly, it’s quite expensive to keep a greenhouse frost-free in that region over the entire duration of winter; you can defray some of the cost by insulating the greenhouse and fastening sheets of greenhouse bubble wrap to the entire structure. It’s also good to build mini greenhouses with polycarbonate sheeting and set them inside the main greenhouse, especially for zone busters and collectors. You can lengthen the growing season significantly by building an insulated foundation and use polycarbonate sheets as a cover. While this might not provide enough warmth for winter cultivation, it’s perfect for storing and overwintering plants, as well as seed starting in early spring.

Greenhouses in the north of Sweden

In colder climates, the greenhouse’s main role is to prolong the growing season. The short but intense summer, with its long days and lighter nights, causes the plants to develop extremely quickly. The time span from when the plants begin to show green to being in full bloom is shorter than in the southern part of Sweden. Many seeds sown later in the season catch up with those sown earlier, thanks to the bright and prolonged natural light; yet even here, it’s still rewarding to start seeds in the greenhouse. During spring and fall you can heat the greenhouse at night, but you should also cover it with straw mats, shade fabric, insulating mats, or woven rugs, and keep the plants inside the greenhouse during the night, tucked in with several layers of white fiber cloth. If you make the sustained effort of covering and uncovering the greenhouse, you’ll limit your heating costs while keeping your plants in good shape.

A greenhouse that needs to be kept frost-free over winter is going to be an expensive greenhouse to run, so a lean-to house or one that is partially in-ground is a cost-effective option. If you cover it with polycarbonate you’ll save even more energy. A simple greenhouse kit made of glass is very expensive to keep heated year round—even if it’s one of the more economical models to buy—because in the north of Sweden you not only need to heat it to be able to cultivate through winter, you also need to keep it sufficiently lit. Keeping a greenhouse in cold climates requires far more work and a larger financial outlay than one situated in the southern part of the country.


A DIY-built cultivation cabinet in polycarbonate is good for propagation and overwintering.

Cultivate or overwinter

There is a big difference between overwintering and cultivating plants in a greenhouse during the winter. The ground has to be warm for cultivation to be successful, and plants need enough light for photosynthesis to take place. Without supplemental light the plant will simply wait, rest and hopefully survive, which is not what cultivating is about. To cultivate means to bring about new growth in the form of new leaves, flowers, and shoots.

Most plants need more light than we can provide naturally if they are to grow in wintertime. Our days are darker and shorter than in the Mediterranean region where many of our most beloved houseplants come from and feel most at home in. To get popular plants like lemon, olive and camellia to truly thrive, they need not just heat but also extra light; they will survive at 8°C to 10°C (46.4°F to 50°F) without extra light, but they will not grow until there is enough light. Their survival is utterly dependent on how much light and heat you give them, and how large an investment you have to make in them is contingent upon where you live, since identical solutions will produce dramatically different results in different areas of the country.


At the top: Attaching bubble wrap. Underneath: The insulating drapes are in place.

Winter insulation

An interesting solution for keeping plants adequately protected in winter is to set up a smaller greenhouse within the main greenhouse. These mini-greenhouses come complete with heating elements and light sources that make it possible to cultivate in winter without it costing a small fortune. An alternative is to build insulated grow cabinets to use for either cultivating or overwintering, depending on where you live and what types of plants you’re dealing with. A growth cabinet can be fashioned from fluted polycarbonate sheets attached to a wooden frame; rigid hinged sheets act like doors, and the house is customized to hold your particular collection of plants. A simpler construction still is rigid, fluted plastic sheets affixed to a storage shelf; the cabinet can have a light fixture fitted to the underside of the shelf and a fan heater placed at the bottom. An overwintering space can also be made up of tented sheets of bubble wrap and insulation sheets on the ground where the pots are placed; complete the whole setup by installing an electronic frost monitor.


The young plants have their own tent with added heat.

Or pick an area of the greenhouse, then insulate and heat only that demarcated section. Plastic greenhouse bubble wrap (which can be bought by the meter or yard) is attached with special fasteners to the greenhouse frame in the fall, and is left on for the duration of the winter. This material is available at greenhouse manufacturers and retailers (see picture on page 158). It takes less energy to heat a single section of the greenhouse instead of the whole space, and if more protection is needed you can build another tent inside the insulated area of the greenhouse. An inner tent made of insulating material is a good complement to bubble wrap, as it prevents heat loss during the night. There are many different solutions out there; try a few of them until you find one that best fits your budget and/or your plants’ needs.

Insulating drapes

Insulating drapes are used to prevent heat from escaping through the greenhouse roof. They’re most commonly used in heated structures, but can also be found in unheated ones. The fabric is set straight across the ceiling—not all the way up to the roof ridge but up to the ceiling joists, then left to hang down the greenhouse walls; special cord attachments help move the panels. The main problem with this method is that it can interfere with the vertical growth pattern of summer plants such as tomatoes and cucumbers, as they cannot make their way up to the ceiling unimpeded.

The very best insulation panels are made from woven plastic with braided aluminum strips (see page 158). Their shiny surface reflects the sun’s rays outwards and the greenhouse heat inwards. There’s a science to choosing insulating curtains, and commercial growers select their panels very carefully. The material can have different spacing between the aluminum strips, the weave’s stitches can be in a range of thicknesses, and colors may vary; you can draw several curtains across in multiple layers for a increased effect. When you buy the curtains from greenhouse manufacturers they’ll be able to advise you on the types of panels that best suit your needs.

Using insulating curtains conserves heat; if you intend to install them, make sure to use them diligently during spring, fall, and winter. They can be moved back and forth depending on the weather, which entails a bit more elbow grease but will ultimately save you a lot of money when it comes to energy costs. Before you decide to buy the curtains, however, consider how often you will actually use them; if you incur the expense, you should make full use of them, and not just hang them for show.

It’s important to hang the insulating curtains correctly so that there’s no gap between the panels when they’re drawn. A gap acts almost like a chimney, making a draft that funnels the warm air out of the greenhouse instead of keeping it inside. In the dead of winter the curtains need to be drawn at all times, day and night. If you want to cultivate during this time, you’ll have to add light in the greenhouse, which will also give off a little bit of heat. The same curtains work as shade fabric during the summer (see page 162).


A carpet with heating coils for plant pots to stand on.

Heating the soil and the air

If you want to grow plants in the wintertime, it’s not enough to insulate the greenhouse with bubble wrap or insulating curtains; you still have to heat the space, and this can be done in a variety of ways. The seeds and the roots in the soil require the most heat, and the seed packet will indicate the temperature needed for the seed to germinate. Ambient air of 20°C (68°F) won’t be of any use to a bean if the ground temperature isn’t at least 15ºC (59°F), which the bean needs in order to germinate. If the soil temperature is only 5°C (41°F) germination will simply not happen. During the growing season the ground soil is naturally warmed in the sun and the air; in winter this source of heat is not enough, which means you’ll need to heat the ground in the greenhouse artificially to give the plant the temperature it needs to survive and grow.

Necessity is the mother of invention

Many greenhouse owners come up with their own ingenious solutions to the challenges of heating their space, just as you will know best what you need and what is available to you. If you have access to free warm water, free firewood, if you can sew curtains or drapes, or if you work as a plumber, a carpenter, or within the packaging industry, you’ll have options otherwise unavailable to others and can thus create your own custom solutions. Early on, underground brick piping was used under greenhouses and hotbeds; they created a network through the greenhouses that linked up to a boiler, which was typically located in a workroom in one of the greenhouses’ gables. The boiler ran day and night when necessary; during winter nights it was the gardeners’ task to keep the fire burning. Hoses that circulate warm water are a modernized incarnation of that type of heating system, and its DIY version is to let all grey water—the waste water from washing dishes, the laundry, and showering—pass through hoses in the greenhouse ground before it ends up in the sewer lines. If your house has a water heating system, you can pull a coil through the greenhouse ground.

Electric coils or heating cables are a flexible, convenient, and fairly common solution for heating the ground (and the air, indirectly). The coils are placed in a layer of sand below the ground bed where the plants are grown, and work much like subfloor heating. They’re typically sold already grounded, complete with tripping mechanism and built-in temperature sensor, and need to be installed by a qualified electrician. Special electrical mats for growing tables or in mini-greenhouse boxes also mimic subfloor heating—you just plug them in and the mats heat up. Cover these mats with sand, and secure your potted plants in the sand. If you have a mat in a mini-green hotbed, it’ll be both hot and humid in that limited amount of space. You can also place your mats on polystyrene sheets directly on the floor, and place your larger pots on top.


Fan heater in a greenhouse with brick base.

Fan heating

The warmth of the ground rises up into the surrounding air, which helps make plants bloom. While the gap between the temperature of the ground soil and the air cannot be too great—there has to be a balance—the temperature of the ground should be your primary concern. Depending on where your greenhouse is located, you can keep it frost-free and overwinter plants successfully by using only a fan heater. As the plants are not being encouraged to actively grow, the heat from a fan heater is enough to overwinter plants that do not survive frost spells, but will not make whatever’s planted in the greenhouse ground start to grow.

There are several ready-made heating solutions available for greenhouses using warm air fans. A fan heater placed on the floor moves the warm air around to heat the inside of the greenhouse. Fan heaters affixed to the walls spread warm air through a perforated plastic hose extended over the entire length of the structure; its advantage is that it’s placed higher up than floor level, thus improving the air circulation. The fan can, depending on its type, work as a frost monitor when used in conjunction with a timer or a temperature gauge.

Space heaters are also commonly used, although they are not intended for use in greenhouses; they are unsafe and can even be deadly. It should be highlighted here that all electrical and heating installations must abide by all current and applicable safety rules and regulations.

Less common in use are heaters powered by kerosene or liquid petroleum (LP) gas. There are several similar types of combustion-fired heaters, their advantage being that they give off carbon dioxide that plants need; however, as the heat burns during the night when the plants are not using photosynthesis, this bonus is somewhat minimal. These kinds of heaters also need to be refilled at short and regular intervals.

Mini hotbeds

Plant propagation requires heat coming up from below, so boxes outfitted with heated mats are ideal for this purpose. When sowing and potting cuttings, germination and rooting take place quicker if there is bottom-fed heat; these same hotbeds can be used for overwintering.


As has been noted earlier, the temperature in the greenhouse is of utmost importance. While you won’t be able to control it very precisely in your hobby greenhouse, you can still monitor and measure it. If you use some type of added heating, you should be able to notice its effect on the plants. What is absolutely non-negotiable is a digital maximum-minimum thermometer (see picture on page 182). It indicates the highest and lowest temperatures as well as the current reading. It’s best to check it every day during spring and fall, and to recalibrate it so you can keep a close eye on any temperature fluctuations. During the summer months you won’t need to reset it or read the thermometer quite as often, as the temperature and weather are most likely more consistent throughout the day and night.

It can be quite an eye-opener to discover the discrepancy between daytime and nighttime temperatures; the sizeable difference can, in fact, often explain a lot about what’s going on with your plants. If you’re methodical and take notes, you may find out, for example, why leaves are turning yellow, are wilting, or simply look odd.

If you don’t have a heated greenhouse but keep plants in there, you’ll need to stay vigilant about monitoring its temperature. If all of a sudden there’s a clear night after a bright early spring day, there’s an increased risk of frost—especially if there’s a full moon—so you’ll need to either switch on a fan heater, or cover the plants and the greenhouse with fabric cover and mats. You can also bring plants inside the house for a night or two. If you decide to cover the plants in the evening, it’s especially important to uncover them in the morning or else they will overheat.

Elevated temperatures aren’t too important for cultivation. Quite the contrary, since many plants don’t do well at all when the ambient air reaches 30°C to 35°C (86°F to 95°F); temps around 20°C to 25°C (68°F to 77°F) are far preferable. Ventilate the greenhouse to keep things in check and make sure the air doesn’t get too humid, especially in the evenings and at night. High humidity makes it easier for fungal diseases like grey mold to take root among the plants, which is why you shouldn’t close all the doors and window vents early in the afternoon in an effort to ‘save’ the heat for the nighttime, because in so doing you’ll only create extra problems.

For that same reason, don’t shower your plants in the late afternoon or in the evening; water them only during the midmorning hours. Fungal diseases favor an atmosphere high in humidity paired with cold nights, which is why tomatoes tend to fall prey to grey mold in the fall.


One way to lower the temperature in the greenhouse is to stop the sunlight from entering it, either by using some shade fabric, sometimes called a shade sail, or with an insulating curtain (which can also be used during the cold part of the year).

Shade fabric is usually a dark green, sparsely woven net. It is relatively light and flexible, and is a bit similar to burlap. In fact, burlap can also be used as shade material, and is eco-friendly. Shadow fabric is synthetic, it won’t rot or mold, and it dries quickly if it gets wet. Burlap is light brown, a bit gummy, and is also quick to dry but is not as durable as shadow fabric.

An insulating curtain is a sparsely woven fabric with added luminous aluminum strips. The advantage of using insulating panels here is that they can do double duty by being used to keep heat in the greenhouse when it gets cold outside.

The curtains can be fastened with clothes pegs to lines, clamped on with special clamps or hung along the greenhouse structure. The fabric is lightweight so it’s easy to deal with. If you prefer a more permanent set-up, string the lines along the edge of the ceiling and the roof ridge. Add a channel for a drawstring at the top of the fabric so you can pull the panel across like a normal house curtain.

Fiber cloth

For shade only, as well as for slight insulation, you can use a white fiber cloth or a row cover. It’s thin and lightweight, and can be draped across plants or attached to the greenhouse frame without any complicated pulley systems. The cloth can be sewn, glued, or taped together in many ways; it can also be washed in the washing machine’s delicate cycle. Use light lace curtains to add an extra touch of ‘pretty’—they make fine shade cloths.


An older way of shutting out the sun was to cover the windows with chalk. A watery mix of special chalk paint was brushed on the outside of the greenhouse panes, the idea being that once it rained the chalk would be washed away and let all the light back into the greenhouse. Warmth and sun in the summer often made for a fairly stable high-pressure system, which could last a few weeks, then ‘nature’ would take care of the rest. Despite its traditional aspect, however, chalking is not a good idea—it’s better to use a translucent curtain—because chalk paint still lets in infrared rays (i.e., the heat), but not the light in the UV-rays.

Shade fabric panels

Whether you’d like shade in the greenhouse or added comfort to an outside room, you can achieve this easily with shade fabric panels. They look a bit like window curtain panels and span the width of the windows. They are made from thin, stiff netting, similar to mosquito netting, and come in light and dark shades. The panel is hung from an aluminum rail and is moveable—as the sun moves, the panel shifts to cover the seating area. You can make these yourself. Old bamboo blinds that roll up and down can be used in the same fashion (see picture on page 153).


This shade curtain also offers some winter protection, as it covers the whole ceiling.


Plants need light, heat, fertilizer, and water in order to live. Water is used during photosynthesis, and without it no plants will grow. You can’t spoil plants by watering them; that they get water every day doesn’t mean that they couldn’t cope if they had to endure a short drought. Were they to receive too little water, they would still survive, but they wouldn’t thrive. Their growth would be stunted or they wouldn’t grow at all, with low yields and fewer flowers.

The idea behind having a greenhouse is to provide plants with a spot where they have an optimal chance to thrive; in order to do so, they need plenty of water. It’s not enough to water big tomato and cucumber plants once a day at the height of summer. They would probably make it and even yield some fruit, but not to the extent that if they had been given enough water.


A shade curtain made of burlap.

It’s important to make sure that plants never suffer from a lack of water or fertilizer. When plants wilt, their growth halts.

Automatic irrigation is by far the simplest way to streamline the maintenance of your plants and ensure adequate watering, and there are systems available in a wide range of sophistication. It’s a great help even if you only have a few large potted plants to deal with, but for someone who cultivates on a large-scale basis it’s most definitely a must-have. For example, cucumbers have somewhat shallow and thin roots but large and lush green foliage. The roots don’t have the strength to absorb the water required to keep the leaves satiated on a warm sunny day. If you don’t use automatic irrigation and only water the plant in the morning and in the evening, the plant will wilt over several hours in the middle of the day. This, in turn, will make many of the budding cucumbers turn yellow and fall off. Plants abort, i.e., they drop their fruit prematurely, when there isn’t enough water and food to grow all of the fruit.


A drip irrigation hose below pepper plants.

Drip irrigation hoses

There are several ways to provide automatic watering to the plants in the greenhouse, but more often than not it’s some type of drip irrigation. The plants receive water either constantly or at regular intervals, during the day and at night. You can use a special hose that seeps water slowly, which is called a soaker hose. A standard-sized foam hose with a built-in pressure reducer is attached to a tap and consistently lets out a small amount of water; the hose is placed alongside the plants growing in the ground. You can’t use this kind of irrigation for pots as the hose needs to lie flat, and the only drawback to the soaker hose is that it can be difficult to judge how much water the plants actually get. However, the method is simple and more or less foolproof; it’s also well suited to outside flower beds. It saves water and the water goes where it is needed; there’s no wasteful spillage, and the plants grow much better when water trickles down slowly to the roots instead of being drenched by a cold shower.

There are also thin plastic hoses with holes that work well at low pressure; the water drips out of small holes or from special seams. Here again it can be difficult to see how much water reaches the plants, but it works well otherwise. These also cannot be used for watering the contents of pots, as they too need to stay on the ground. What you can do, however, is bury them in the ground and the water will reach down to the roots—their intended target.


Several irrigation drips can be placed where plants need a lot of water.

Automatic drip irrigation

An automatic drip irrigation system is typically made up of a network of tiny thin hoses attached to slightly larger hoses, which in turn are attached to a main feeder hose. This feeder hose is connected to a faucet, and by opening the faucet to varying degrees, you can control the flow of water that’s dispersed.

There are also setups equipped with a small temperature gauge, which is connected between the drip irrigation attachment and the faucet. As temperatures decrease, the rate of dripping water slows down; inversely, when temperatures rise, the amount of water dripping increases.

Water seeps slowly into the ground from the thinnest hoses. They are secured by tubing hold-downs in plastic that are pushed into the ground next to the plants.

One or several drips can be placed at each plant both in pots and directly in the ground. From the main hose you can turn on many drip spots simultaneously; however, the pressure has a tendency to decrease the further the drip point is from the faucet. You can place a measuring cup underneath the drip to check the volume of a drip over a twenty-four-hour period, and can compensate for the low pressure by placing several drips per plant in the affected area.

Bringing water to the greenhouse

Bringing water to the greenhouse needn’t be complicated. A faucet placed by the greenhouse door is convenient, but if water is needed only for summer usage you can simply attach a heavily reinforced hose from a self-draining water source to the greenhouse, and it can rest on the ground. The hoses can be connected by simple plastic hose connections, although they should be stored inside over winter as they can become brittle in the cold, and split or become leaky. Often all you need to do is replace the washer in the hose connection and it will be tight again, ready to use one more season; washers can be found at most hardware stores.


Drip irrigation is simple and convenient.

The situation becomes more challenging if you need year-round irrigation. You’ll need the services of a plumber, as the feeder hose needs to be buried in the ground below the frost line and it needs to be properly connected. Hoses for your water supply in summer can be set in quite shallow, and be attached by plastic quick-disconnect adaptors if you don’t want them to be visible at all times.

Hoses are available in a wide range of grades. A hose that undergoes constant water pressure needs to be reinforced, because when the hose is exposed to sunshine the water inside heats up and this causes the hose to swell; in the end the hose will burst if it isn’t reinforced (alternatively, the plastic adaptors could fail and shoot off like fireworks). A good quality hose is expensive but will last longer; a cheaper hose that twists easily and cuts off the water supply is not a good option.

The greenhouse’s thin water transport, drip hoses and attendant accessories for the drip irrigation system are plenty affordable, so buy enough of them to have on hand for future use.

The same drip irrigation system and hoses can be used for pots on the deck or patio if you don’t want to bother with repeated watering; simply connect them to reinforced feeding hoses that are attached to a water source. Hanging baskets are a bit more problematic to water, but you can purchase special plastic water—supply containers that hold several quarts of water.

Automatic timer

Both the hose and the drip irrigation system can be set up on a clock or timer. It works a bit like a kitchen timer, with a special irrigation clock that turns the water on and off at set times, at even intervals, over a span of twenty-four hours. It can even be pre-set for how many times you’d like it to drip each time it is turned on. There are yet more complex systems available on the market, and the more complicated the system, the more expensive it is. That being said, an automatic clock or timer doesn’t really offer up any advantages over a simple drip irrigation system that works perfectly well, day and night, without any disruptions. Further, the more complicated the system, the bigger the risk something will go awry; an automatic system takes time to set just right, whether it’s to be in use twenty-four hours a day, or be set on a timer.

Safe vacation solution

It might not feel prudent to leave a drip system on and unattended while going on vacation. Hoses can burst, attachments can fail, or someone might simply stumble over the hose and accidentally disconnect it. Fortunately, there are other options to having the hose connected to a faucet. Drip irrigation can also be connected to a water barrel, where the pressure is controlled by gravity. This is enough water for a few days; you’ll eventually need to fill the barrel again. If an attachment fails or something else occurs, the loss of water will be limited to the contents of the barrel, and the leak will probably not happen in the greenhouse. For just a few days’ usage there are special kits and drip-watering bags; i.e., plastic bags that can be filled with water and hung on the greenhouse wall, to which a few drip points can be attached. The water won’t last long but it will be enough to tide the plants over for a long weekend.

Automatic fertilizing

Plants need nutrients to grow (see page 26). They can either be added to the soil or to the irrigation water. Hydromat, a Danish product, is a complete irrigation/fertilizing system and a very good investment. In addition to drip hoses, a holder, a feeder hoses, and a pressure valve regulator, the kit comes with a container for fertilizer. The container is mounted high up in the greenhouse and operates through water pressure and siphoning effect. The container is filled with liquid fertilizer, which is meted out along with the water as the water flows through the container, so that the plants are fed continuously. The Hydromat can also be adjusted to control how much water goes to each plant; you’ll need to program the amount of water needed to drip over a twenty-four-hour period. You can choose, to a certain extent, the number of drip points that need to be installed. You can irrigate some plants more than others by putting in extra drip points. If different plants, such as tomatoes and cucumbers, are connected to the drip, it’s usually the cucumber that needs the most water, so attach two drips to each cucumber plant and one to each of the tomatoes.

Keeping an eye on it all

It takes about a week to program all the drip settings for the plants. Start well before going on vacation so everything is working to your satisfaction on the day of your departure. Do a tour of the greenhouse once a day at the beginning and feel the soil; check for pooling water or if there is a proliferation of fungus gnats anywhere. Fungus gnats are small, fast-moving flies that you’ll find on humid soil. If a cloud rises up from the plant as you approach it, it’s usually a sign of these flies, and is an indication the air in the greenhouse is too damp.

A rough guesstimate is that plants should be given approximately 1/2 liter to 1 1/2 liter (1 to 1 1/2 quarts) liquid a day per twenty-four-hour period, no matter how many drips are generated. Plants that are not given enough water will wilt, but if the soil is very wet and the plant is wilting anyway, the problem is probably due to excess water. The soil surface between the drip points should be dry; plants absorb only what they need, but that doesn’t mean that the soil in the pot or in the ground can’t be too humid. Roots don’t fare well in excess moisture.

Plants will need more humidity at the height of summer than during spring and late fall. You’ll need to increase the irrigation when temperatures rise and there is more sunshine; also, big plants require more water than smaller ones. By late summer you’ll need to decrease irrigation, or fungus and mold problems might flare up.


A water source to which a hose can be attached.

Showering and misting

A spray nozzle attachment to the garden hose is a common tool in many gardens; while the spray nozzle is not suitable for watering the plants, it is acceptable for showering them if the spray is set to ‘mist.’ It can also be beneficial on very hot days to shower the ceiling, floor, walls and plants in the greenhouse, as it lowers the temperature and allows the plants to breathe easier. The risk for aphid and spider mite attacks is substantially reduced, as spraying with water is the best way to combat these pests—far better than using chemical insecticides. If the plants are wilting, a shower will also help them to perk up, but the water should be lukewarm, never ice cold.

Misting spray nozzles are mini sprays that attach like pushpins to a hard plastic hose (see picture on page 76).

When the water is turned on, a fine mist is dispersed through the little spray nozzles. Nozzle misting is perfect for cooling down vegetables and improving the pollination of tomatoes. There are ready-made systems available for purchase, and the rigid plastic hose is affixed to the greenhouse ceiling, making the spray come down like a fine fog, which gives the water enough time to warm up before it sprinkles the plants, and doesn’t fall like a cold shower. This system can also be connected to a clock or timer, but misting is not done as often as watering. It’s enough to manually turn the mist on a few times in the middle of the day as the temperature climbs. Misting is not needed on cold overcast days, and never mist in the afternoon or evening, irrespective of temperature (this is why manual control is preferable to an automatic misting function).

Extra water and nutrients

Even if you have an automatic irrigation system, complete with fertilizer, it’s not always enough, so it’s always a good idea to check the greenhouse, watering can in hand, to give needy plants some extra watered-down fertilizer, which might be a plant in a drafty corner or in an excessively sunny spot. Also, check for the signs of malnutrition, which is most obvious in new plant growth. It’s perfectly okay for old leaves to turn yellow, but new growth on the top should be green and fresh looking. If it looks tired, try feeding it some nutrients in the form of Blomstra (or equivalent fertilizer) at 1 ml per quart of water for a few days, and check back to see if the plant is perking up.


A Hydromat system complete with protective polystyrene ‘hat,’ to avoid the growth of mold in the container.

The drip feed can dam up so no water comes through. Check the drip frequency by placing a measuring cup under the drip next to the wilting plant. Also, check under the plant to make sure the soil is moist; if the soil is dry, the drip might be clogged up due to mineral deposits or fertilizer. Remove the drip and blow through the hoses to clear them. The fertilizer container might not be working, or the fertilizer might be used up, since it needs to be refilled several times during the growing season. Be extra attentive while checking the irrigation system, not only to secure the harvest but also because plants that are left to dry out are more susceptible to attacks by aphids and other pests.


It’s a good idea to install shelving that can be dismantled and moved around in the greenhouse. When plants grow tall, simply remove the top shelf.