Greenhouse Vegetable Gardening (2015)

For the greenhouse to feel cozy in wintertime, you’ll have to add both heating and lighting to the room. However, if the greenhouse is only to be used during the warmer seasons, it only needs to be heated in order to keep the non-hardy plants alive.

By making use of a variety of different plants—from hardy climbers to delicate houseplants—you’ll be able to create a leafy, pleasing atmosphere in the greenhouse all year round. No plants flower or are at their best every season, so you’ll want to grow many different plants and flowers to ensure some color or green throughout the year. Many ornamental plants are perennials but are not hardy, and some are evergreens. Many originate from countries where the sun shines brightly and the summers are warm, and the winters slightly cool. It’s really a misnomer to call them houseplants, then, as they wouldn’t be able to survive inside our houses in the muted light of winter—they need a brightly lit, heated greenhouse. Citrus, camellia, myrtle, pomegranate, rosemary, bay leaf, olive tree and figs are such plants. They would lend a distinctly Mediterranean air to the greenhouse.

Other perennials—even those able to thrive outside year round in our climate—can supply added splendor to the greenhouse. Climbers have lush foliage yet don’t take up a lot of floor space. The grape vine is a traditional climbing plant grown in greenhouses—one of its draws being that it’s relatively hardy and can survive winter in an unheated enclosure. It also produces beautiful leaves that provide some well-needed shade on sunny summer days. The flowering clematis is also hardy and can be planted in-ground. There are cultivars that flower in early spring, summer and fall, so if you sow a combination of early and late blooming plants, you’ll be able to enjoy a variety of gorgeous flowers continuously throughout the growing season. The hardiness of roses can occasionally be a bit unpredictable, but thanks to the shelter of the greenhouse you can rely on seeing many more of those flowers come up (as long as no deer drop in for a visit). Potted roses will also bloom earlier and longer. Bougainvillea and passionflower need warmth and thrive up near the ceiling—they will suffer if kept in the living room’s dim winter light—and require plenty of light and a frost-free place to stay in the winter. This applies to the potato vine, too, which is a sturdy, vigorous climber with white flowers. Temperatures cannot dip below 0°C (32°F) when growing such exotic climbers; the air should be kept around 5°C to 10°C (41°F to 50°F), even if these plants can withstand one or two chillier nights. They’re ideal companions to citrus plants.

Spring delights

While each season has its own particular charm, you can force spring flowers into bloom earlier in the greenhouse. Whether they’re your own forced bulbs or come from the nursery, they all provide great bursts of color when grown in pots, and last longer in the greenhouse, thanks to its cooler nighttime temperature. Demanding light-seekers, such as primulas and the first early geraniums, also bring delight whether they’re from your own cultivation or store bought plants. Many of the summer flowers ready for transplanting outside create as pleasant a room as does the first humble pot of springtime perennials, of which you can find a wide selection to choose from at garden centers. Perennials overwintered in the greenhouse will also flower earlier. Peaches, nectarines and apricots—all showy with pretty pink flowers in the spring—can be grown in large containers in the greenhouse. These plants are hardy and don’t need a frost-free room since they go dormant in the winter. On the calendar, alpine clematis is the first of the climbers to flower—displaying lovely blue and pink blooms.

Plants waiting to be transplanted out enjoy the company of store-bought geraniums.

Flowers in the summertime

Many plants do better in a warm greenhouse than outside. The enclosure gives us ample opportunity to try all different kinds of flowers in order to find out which ones we like best. Blue potato bush and lantana both put on a beautiful flower show. Both can be trained up as trees, providing a charming accent to the greenhouse.

Small trees (created by training up) such as hibiscus, cape mallow and other houseplants, can sometimes be found at garden centers. They’re well worth trying out, even though there’s no guarantee they’ll survive the winter.

Heliotrope releases its sweet, heady scent on summer evenings. Many salvias are perennials, although they’re not hardy plants; they come in many colors and shapes. Some grow almost to the size of a small tree. Luminous gentian sage and scented red pineapple sage, violet mealycup sage and many more beauties like petunias, marguerite daisy and more traditional bedding plants offer up a fantastic array of rich colors. Some plants like browallia (sapphire flower), coleus, New Guinea impatiens and garden impatiens can be used both as bedding plants and houseplants (which also benefit largely from staying in the greenhouse during the summer. See more on houseplants, chapter 14, page 129.)

Water helps to cool the greenhouse. Myrtle thrives year round as long as there is no risk of frost.

Annual climbing vines provide the lush, welcoming, flower-filled atmosphere we imagine a greenhouse to have. Morning glories in all their bright colors, scented sweet peas, and cup and saucer vines in dense foliage, all grow perfectly in this environment. Start them from seed and leave them to grow. Plant and train them in large pots; the larger the containers, the denser their flowers and foliage will become, and the easier the plants will be to care for. Do not move small plants into pots that are too large at the outset; they need to be potted up into successively bigger pots during their growth during spring.

Scarlet runner beans, sugar snap peas and haricots verts (small French green beans) are more than just pretty flowers—they’re also edible. Their leaves are bountiful, their flowers decorative and some beans even have different colored foliage. They’re very easy to care for, and rewarding in all their simplicity. The climbing snapdragon starts out slightly frail, but is wonderful to behold in full bloom—pink, blue, and wine-red graceful beauty against the greenery. Climbing garden nasturtium, canary creeper and gloriosa lilies add joyful and luxuriant blooms in shades of red and yellow.

The garden mum will flower for a long time if kept in the greenhouse.

Fall splendor

The colors of summer flowers continue to delight us into the fall. They’ll bloom until killed off by the first frost—unlike perennials, which simply go dormant. Roses often stubbornly continue to produce blooms—that’s why roses are often killed by the outside cold. They don’t adapt well to winter temperatures and continue to grow, so keep them in the greenhouse for extra protection against the elements. Once fall arrives, potted chrysanthemums—mums—and asters can increase your flower power. These plants have lovely blooms and are well suited to use in bunches of cut flowers.

It’s questionable whether chrysanthemums are hardy north of Skåne, the southernmost province in Sweden. You can dig up the complete plant and store it alongside dahlia roots from year to year. This is frequently done, even though chrysanthemums do occasionally make it through the winters outside in Skåne.

The last rose of summer bravely faces the elements.

Late New York asters seldom have time to reach flowering stage in the north of Sweden because of the early onset of the cold. If you place them in the greenhouse, however, you’ll get to enjoy them far longer. Even dahlias, when given a space in the greenhouse, will continue to flower long past first frost.

Early-flowering plants—like peaches—need help with pollination.

Taking care of flowering plants

As there will be a variety of plants in your greenhouse, their care will require a few different approaches. The common denominator, however, is that during the dark and cold part of the year, watering should be done sparingly, and fertilization even less. No plants should dry out completely, nor should they be tempted to grow from liberal watering and feeding. You must also carefully tend to the pots to make sure they, and the area around them, are kept clean and tidy. Remove all the dead leaves and other decaying plant material so they don’t help grey mold set in. An integral part of every fall’s big gardening clean up routine is to shower and tidy up your plants.

Small trees

Peaches, apricots and nectarines are pretty simple to keep in the greenhouse. They start off as small trees for planting in large pots, and they can survive some cold. Not weeklong temperatures of -20°C (-4ºF), mind you, but temperatures slightly below 0°C (32°F) won’t cause any havoc. The trees’ pink flowers appear very early, which is one of the reasons these plants don’t fare well further up in the north of Sweden. The plant itself survives the cold but the flowers are destroyed at around 0°C (32°F), a very common outside temperature at which flowers tend to make their appearance. The greenhouse protects the flowers with its milder ambient temperature, but a new problem emerges due to missing, yet vital, actors in the growth process: insects. In order to remedy this, we need to step in and play the part of the bumblebee, and pollinate the flowers by going with a small watercolor brush from flower to flower to transmit the necessary pollen. You don’t need to add another plant, but the pollen must reach the pistil.

Potted plants need additional nutrition by way of regular fertilizing. Fertilize at each watering while the trees are green and in bloom, and stop fertilizing once the harvest is over. Make sure the containers are watered even in winter—the soil should not to be too wet, but it should not turn to dust, either. Bay leaf, rosemary, olive and fig trees are other good candidates for the greenhouse. Even they can withstand a temporary cool down, but nothing drastic or prolonged. Fig trees drop their leaves in winter but the rest of the trees stay green, requiring only light and to be watered sparingly. Check on the plants to make sure the soil doesn’t dry out, and irrigate as needed. When the seasons change and it becomes lighter and warmer outside, it’ll be time to start adding fertilizer to each watering session.

Citrus—a demanding plant throughout the year

Citrus plants are a real challenge to grow and maintain. They’re certainly lovely to behold and a beautiful addition to the greenhouse, but they need a lot of light in winter, so you’ll need to pick their spot with extra care. Temperatures can stay around 5°C to 10°C (41°F to 50°F), but an adequate source of light is of utmost importance. There are many different kinds of citrus trees, and their common characteristic is that they can flower and bear fruit simultaneously. The calamandin (calamansi, or Philippine lime) is considered the easiest citrus to cultivate, but like all citrus it needs to be planted in nutrient-rich soil. Use specially formulated citrus soil, i.e., mildly acidic sandy loam. Make sure to water the plant even in winter—sparingly, but not to the point where the soil dries out. Citrus needs special fertilizer during the brighter, warmer season.

Peach, nectarine and apricot trees flower very early. If left in the shelter of the greenhouse, their blooms will last a long time.

Citrus plants are beautiful in bloom, and produce attractive fruit that remain on the branches for many months.

Other plants, such as pomegranate and crenate orchid cactus, also thrive in a slight winter chill. They prefer their conditions to be frost-free and with plenty of light, and temperatures between 5°C and 10°C (41°F to 50°F), although they can handle chillier rooms in short spells.

The myrtle is an old-world houseplant that has nearly vanished due to our overheated homes. It prefers temperatures around 5°C to 10°C (41°F to 50°F), and it grows well together with camellia and geraniums. Camellias are a bit special, since they flower so early—before Christmas if they are given extra light, but more typically in February. Their beautiful green glossy leaves show that they’re related to the tea bush; they’re an adornment year round. There is an exceptionally wide array of cultivars to choose from, and it’s a pure delight to see the blooms appear. Camellias prefer acidic soil; they should be watered with calcium-free water (rainwater is excellent) and they should be given rhododendron fertilizer.

A good way to successfully maintain demanding plants is to install permanent diffused lighting in the greenhouse. This will help them to flower earlier and more profusely; they’ll grow better and appear more abundant in the winter.

Flowering roses

Roses are probably not the best plants to keep in the greenhouse, as their many prickles (thorns) create problems. Modern roses, however, bloom with gusto and without peer to any other perennial plant. They can produce flowers almost year round, but in order to do this they need diffused lighting. When a shoot has flowered, it takes between five and six weeks before it blooms again. Potted roses that have been trained to standard can therefore provide a real splash of color for a long time. Roses can endure a cold snap during the winter, but it’s better to keep the temperature around 0°C (32°F). Stay vigilant in early spring: roses have no innate growth control, so if it starts getting warm they’ll start turning green. The greenhouse warms up early, but can also suddenly experience a cold snap. If the temperature dips below 0°C (32°F), young rose leaves run the risk of freezing, and if this happens it’ll be quite a while before new leaves appear. As soon as the plant shows some green, it’s high time to start watering and fertilizing.

Camellias in many colors and shapes thrive in the greenhouse. They’ll cope with temperatures a few degrees below freezing, but only for a few days. They’ll need extra heat to survive, and diffused light for an early bloom.

Tempting sweet-tasting grapes ripen in late summer.

Grapes

A grapevine is a vigorous climber, bringing forth luxurious foliage and delicious fruits. The best grapes for growing in a greenhouse are the ‘vitis vinifera’ species, a ‘true’ grape cultivar. But there are also many other types of grapes—common ones being ‘précoce de maligre,’ which are small and green, and pinot noir, which are quite hardy. There are also new specialty grapes, such as solaris and nero which you can find in garden centers, and they can be used for wine making or enjoyed as dessert grapes. These types are marketed predominantly for greenhouse cultivation, or for growing outside in warm areas in the southern provinces of Sweden.

Hardier and longer lasting than the common grape are the fox grapes. These are vitis hybrids, and are better able to cope with the cold and are more disease resistant. Their grapes are small but very tasty, and they can be grown quite far up north in Sweden. The labrusca grape could be a contender in the greenhouse for someone who lives in a really cold climate, as the vine can take on temperatures as low as -25ºC (-13°F). It’s important to plant the vine in acidic soil or it will not grow. Fertilize with rhododendron fertilizer during spring and early summer. The rest of the time, simply treat it like a common grapevine.

The grape vine provides needed shade on sunny summer days.

The grapevine is fairly hardy and can be planted directly in the ground. It’s an energetic grower and can become very large, but can be pruned and kept small like in a French vineyard. If the ultimate goal is to have plenty of tasty grapes, it’s best to prune the vine hard; if you’re after the lush greenery, however, less pruning will do. The grapevine can overtake the whole greenhouse ceiling and its foliage can cast a dark shadow; however, the leaves open quite late in the season and it’s wonderful to see the vine grow a canopy above the coffee table. Plant the grapevine along the short end of the greenhouse—if it’s a smaller, 10 m² to 15 m² 107.65 to 161.45 ft sq. greenhouse. It’s not a good idea to grow a grapevine in a smaller house if you want to plant other things too, since the vine will take up a lot of space. You can lay slabs very closely; the area of soil for the grapevine needn’t measure more than 50 cm x 50 cm (1.64 x 1.64 ft). Dig a hole, 50 cm (1.64 ft) deep and 60 cm x 60 cm (2 x 2 ft) wide. Fill it with good soil, place the vine in it, water it and then add more soil until the plant is level with the ground. Water the vine regularly through the year right after planting. Beyond that, the grapevine should do well on its own as long as the ground soil is not too dry or sandy.

You can line the sides of the hole that you’ve dug with a root barrier in order to prevent the grapevine from encroaching on the surrounding beds. Alternatively, you can plant the vine in a box or in a planter atop a hard surface, but for this you’ll need a very large container.

Another option is to plant the grapevine outside the greenhouse and train the vines in under the base. A root barrier placed between the grapevine’s planting spot and the greenhouse will stop the roots from forcing their way into the greenhouse soil. This works especially well if you live in a milder climate and want to grow other plants in the greenhouse. Grapevine roots spread far—their growth habit is energetic. Nothing is done to the vine’s brown main stem, depending on when you plant it, and on what the plant looks like. Green shoots that start to grow will eventually be pinched or pruned off, but not during the first summer.

Surround the grapevine roots with a root barrier to prevent them from taking over the cucumber and tomato beds.

Pruning grapevines

Late in the fall, the grapevine will be pruned down to two or three buds on the brown main stem. The shoots that have emerged from these buds are also to be pruned away, leaving only one or two buds. This pruning is repeated in late fall each year, leaving the next bud. The vine’s height increases by one bud each year, and the side branches will also increase by a single bud. This ends up giving the vine the look of a brown, knotty Christmas tree, and that’s perfectly OK for a vine that’s going to grow next to the greenhouse gable.

The vine’s green shoots are pruned every summer. When the plant begins to flower, all the shoots that produce one or two buds from the flower cluster are removed. Soon, new shoots grow on the leaf axils and they are pinched off above one bud. This pinching is done several times over the summer. The leaves are thrown on the compost pile. Shoots that don’t flower are pinched off leaving one bud, or none at all. If there are a lot of grapes, some of them are removed, too. To develop into bunches of large sweet grapes, they need to hang freely and unobstructed so that the sun’s rays can reach them.

There are other ways to train a grapevine. You don’t have to prune the main stem—you can train it vertically towards the roof ridge or drape it over the crossbars in the greenhouse. It’s tied up there as a sort of main stem, and the side shoots growing along the stem are cut off instead. This might actually be a better idea than to grow the vine alongside the gable, but check carefully first to see that the vine isn’t blocking the access to and the opening of window vents. The vine might make it a bit awkward to work around mesh awnings, insulation material or plastic bubble coverings, but there are always ways to deal with that.

The grapevine leafs out quite late. Thus, its shadow doesn’t impede the greenhouse light when it’s most needed—in the springtime.