Greenhouse Vegetable Gardening (2015)
SPRING FLOWERS, BULBS, AND PERENNIALS
Even if your greenhouse doesn’t have a heating system, you can still take advantage of the sun’s warmth to force early blooms, and to overwinter newly sown perennials. That little extra heat nudges the plants into flowering before the flowers in the garden are ready.
There are many more flowering plants you can keep outside in containers than you’d think when looking at the selection in a garden center. There are perennials such as lawn daisy and columbine, bedding plants like pansies, or early spring potted bulbs. You can spread the joy of flowers through spring if you grow them yourself—you’ll be able to enjoy a first round of early blooms in containers and flower beds before classic summer flowers are even put outside.
Typical bedding plants aren’t frost hardy, so they can’t be planted out at any old time in the spring. If you opt for hardy, early varieties, you’ll have flowers several weeks early. Don’t worry if timing is a bit off, if the spring is long and icy cold and you end up not being able to plant anything—even if the flowers go out late, they will still flower as they normally would; you just won’t have them as early as you’d hoped.
To ensure that your plants flower early, you’ll need to begin the process early. The trick is to get a head start by sowing and planting flowers in containers late in summer of the preceding year; then they should be kept protected and preferably frost-free over winter. The early sun’s warmth together with a frost-free environment (thanks to an automatic frost monitor) then forces the plants to start flowering. As they will have been growing in a pretty chilly greenhouse up until then, they’ll be hardy and can be placed outside even if the temperature dips below freezing during the night. Generally, a few degrees below freezing will be OK; hardened-off pansies can tolerate temperatures as low as -5°C (23°F). The growing part is easy. If there’s any hitch along the way, it would be in finding seeds in late summer and fall, so try to buy the seed in the spring when selection is at its widest.
Sowing and overwintering
The most common spring flower is the pansy. It will flower in the year following sowing, and then it dies down and can’t be overwintered outside. For pansies to flower early in spring, they need to be sown the preceding August. Commercial growers typically sow their plants on the same side of New Year’s that they want blooms, since they’re after the flowers, but they do have greater means to guide the climate in which to grow their plants. If you don’t have the benefit of that type of environment, you’ll have to follow the rules of growing flowers according to the part of the country in which you live. There are many early spring flowers that are cultivated this way; they’re sown in August-September and then left to grow to maturity. They’re overwintered in the greenhouse, or perhaps in a cold frame covered with insulating fabric. When the spring sun heats up the greenhouse, any insulation material must be removed; the plants are then treated as per usual—with watering and fertilizing.
The pansy is an early flowering biennial plant. It can be grown in the greenhouse to provide early plantings for the outdoors.
How to’s—Sowing for spring blooms
♦ Sow as usual (see chapter 4, page 29). Put the sown pots and trays in the greenhouse.
It’s important to follow sowing and growing instructions printed on the seed packet. Reputable seed vendors might also include a separate instruction sheet containing information about sowing and different plants’ requirements.
♦ Pot up the seedlings in individual pots with good soil.
♦ Place the pots outside under a protective roof. When fall and cold arrive, bring the pots inside the greenhouse. The plants still need a little time to grow before winter sets in.
♦ The plants overwinter in the greenhouse in their pots.
♦ Depending on the type of plant you’re growing, it might be necessary to provide some extra heat to keep the greenhouse frost-free during the winter.
♦ By collecting all your plants in an insulated growing cabinet, you won’t have to heat the whole greenhouse. You can hang plastic bubble wrap over an open shelf, tape the sides down and heat the inside with a small fan placed underneath the shelf. The plants can also be covered and tucked in with loose layers of fiber cloth. The fiber cloth is very lightweight so it won’t damage or weigh down the potted plants.
♦ When the days begin to get lighter and warmer in the spring, remove the covers and let the plants continue to develop. The right time to take the covers off will depend largely on how early or late the spring turns out to be. For flowers to bloom earlier than when planted outside, you might have to provide some warmth at nighttime. Air out the space during the daytime, since no plants do well in 40°C (104°F) in the day followed by cold at night.
Primroses come in many colors and shapes. Border or garden auricula is an old-timer that was once very popular.
Select or reject
Pansies are biennials, which means that they only flower during a single spring. They come in a great variety of colors, types and shapes—pink, apricot, brown-violet and bi-colors are but a few examples of pansies that have come on the market in the past few years. In Sweden, however, the seed selection is not very large, due to the fact that not many people grow their pansies from seed. English seed catalogs, by contrast, are chock-full of delicate dainties. Since spring is long and arrives early in the United Kingdom, the English have made it their tradition to plant out some early spring flowers before summer’s resplendence. To meet the demand for these early plantings, their seed availability is suitably large.
It’s very rewarding to plant miniature pansies. They’re close relatives of the heart’s easy (wild pansy) and tufted (or horned) pansies, which are occasionally biennial. They’re hardier and easier for hobby gardeners to grow successfully. They sprout many small flowers instead of a fewer large ones, which makes them less sensitive to inclement weather. Their seeds need light to germinate, so they cannot be covered with soil; they must be grown in a mini greenhouse.
Lawn daisies can survive a few degrees below freezing. Aside from sowing them in August, you can also dig up small plants that grow outside and put them in pots (this should also be done in August). Commercial growers overwinter lawn daisies in a frost-free environment at 8°C to 10°C (46.4°F to 50°F) but they are hardy to lower temperatures too. For earlier flowers, however, you must provide them with extra heat—around 10°C (50°F) in the daytime and somewhat cooler at night. Even lawn daisies can be purchased in seed form from well-stocked English seed vendors.
Forget-me-nots should preferably be housed in a frost-free environment to make them flower early. They come in many beautiful shades of blue, but also in pink and white. They are exquisite as companion plantings to pansies and early tulips. Together with lawn daisies, they make a flowering meadow of pink and blue. The forget-me-not seeds need light to germinate.
Green plants, sown in late summer, overwinter in the greenhouse under an insulating cover. Spring sun and the heat from a fan forces them into bloom in early spring. If you grow your own pansies, you’ll have many containers full of spring flowers.
The primrose is often simply called primula, even in Swedish. It has many types and hybrids, and it’s a common houseplant during winter months. You’ll find primroses at garden centers in fall and winter, and it is quite cold hardy. If you don’t want to grow them yourself, buy the plants and harden them off so they’re ready to plant out in early spring along with other spring flowers. They look very good together with miniature pansies. The primrose is also sown in the fall, but it’s advisable not to give it heat too soon—if it starts growing and budding too early, the flowers will be wan and pale looking. The light in January is not strong enough, so it’s better to wait awhile—unless you have growth lights and heat, in which case they can be started. Like forget-me-nots, primroses need light to germinate, so don’t cover them with soil.
The wallflower is a plant that was very commonly cultivated at the beginning of the 1900s. They were used as spring houseplants both indoors and out. Sown in April, they were planted out as soon as they got big enough. Around October, whole big plants were dug up—soil and all—and were transferred to the greenhouse, where they were kept in a cool, frost-free environment until it was time to force them into flower. At this point, the greenhouse was heated up to around 12°C to 15°C (53.6°F to 59ºF)—but not too early in the New Year, as the plants needed adequate (stronger) light to produce attractive flowers.
Today there are newer versions of the wallflower that are easier and faster to grow. They can be cultivated like pansies, and they even tolerate a cold spell pretty well. Many types, but not all, have an attractive scent. English seed vendors sell wallflowers in assorted colors, and they come in all shades except blue.
It can be a good thing to grow some ‘common’ perennials, and to force them into early bloom so they can be used as outside bedding plants. However, in order to do this, you’ll have to provide some nighttime heat in the greenhouse so the temperature stays around 5°C to 8°C (23°F to 46.4°F). Why not give it a go? Columbines are another popular example, and there are many new beautiful varieties with upturned flowers.
The wallflower is an old-fashioned spring flower.
Splendor of the bulb
Bulbs are some of the easiest of all plants to grow. Many flower by themselves very early in the spring. The flower and leaves are ready in the bulb, just waiting for a cold spell followed by a little bit of warmth and light in spring. The most difficult part of growing bulbs is to actually provide an adequate cold snap, and the bulbs need an early winter in order to flower early. For them, winter is when the temperature is at 7°C to 8°C (44.6°F to 46.4°F) and lasts for about ten weeks—although the specific time will depend on what kind of plant you’re dealing with, and also to a certain degree on what the previous summer was like. Commercial growers can offer us flowering tulips in January because they place their bulbs in cold storage as early as September.
Small bulbs such as snowdrops, grape hyacinth, winter aconites and crocus are the simplest plants to force into flower. They don’t need very much heat and light to start growing; they grow quickly and will flower even if it’s cold. They will flower outside even in the chill of early spring—they’re used to the cold and are hardy even at temperatures below freezing. You can also plant them closely together in regular-sized pots to grow many flowers in a small space. As the pot isn’t very big, it’s easy to insert it into a decorative container or plant it in a pot outside.
Daffodils and narcissus are also quite rewarding to force, and you can even make it work with wild (botanical) tulips. Daffodil bulbs are rather large and need deeper soil to be able to stand securely. The result often looks sprawling and stiff so you should plant the bulbs in layers to get more flowers, but the pot still needs to be deep and wide.
Bulbs that flower in time for Christmas have been specially groomed to do so. The bulb itself is heat-treated in a special way so that it only requires a very short period of cold to reach blooming stage.
Christmas Hyacinths are planted on the 15th of September and then kept cool and dark until around the 25th of November. They are then forced in light and at room temperature so that they flower in time for Christmas.
Paperwhite narcissus (daffodil) can be forced to bloom in five weeks without a cold period, and the same goes for amaryllis if it’s specially treated, or if it’s a South African variety. If the temperature is kept steady, you can force them in the greenhouse where it’s probably a little lighter than in the house.
Large tulips and hyacinths are much trickier to handle. They must be kept at 15°C to 18°C (59°F to 64.4°F) and preferably in extra light during the forcing period to blossom beautifully. Hyacinths require even warmer temperatures—around 20°C to 22°C (68°F to 71.6°F—to get going, and then they need a steady 15°C (59°F) during the growing period. The forcing time for adequately chilled tulip and hyacinth bulbs is three to four weeks. Commercial growers use light-treatment on tulips to impart color to the flowers and leaves, so if you start forcing the flowers too early when the light is weak, you will end up with a spindly and pallid flower.
How to’s—Planting bulbs
♦ The bulbs are planted in pots in September. They are placed on a 5 to 10 cm (2” to 4”) layer of planting soil.
♦ Layer the bulbs in several tiers for many closely spaced flowers.
♦ Cover the bulbs with some soil, or preferably with fine sand. The sand is to cover the bulb.
♦ Water the pot until the sand is thoroughly wet. The bulbs should grow roots in the pot, but not start sending up leaves. You will need to check on the bulbs, and water a few times during the fall; the soil should not dry out or stay too wet.
♦ If you don’t have access to cold storage, place the potted bulbs outside when it’s cool—preferably around 7°C to 8°C (44.6°F to 46.4°F), but under cover. When it gets colder outside, move the pots into the greenhouse.
♦ When the bulbs are covered for winter, their soil needs to remain slightly moist. The bulbs need some humidity to grow, but they can’t stand in wet soil over winter because they’ll start rotting.
♦ Dig holes in the ground of the greenhouse and lower the pots into them; cover the pots with leaves or other types of insulation. The soil in the pots should not be allowed to freeze. Mark (with flags, for example) where the pots are placed.
Cover the buried pots with sand instead of soil, as it’ll be easier to rinse sand off the pots when they’re dug up in time to start forcing the flower.
♦ Sometime in the New Year, move the bulb pots into the warmth. They also need light to prevent them from growing tall and gangly, so they’ll fare best in a frost-free greenhouse or an outside room. Water the bulbs with lukewarm water when they’re being moved. After this, do not water them until they start budding—too much water gives rise to long lush leaves and stems that are easily broken.
Plant the bulbs close together for best effect.
Growing perennials from seed
Many garden perennials, in addition to trees and bushes, are propagated through seed. Others such as rock cress and phlox can be propagated through digging up the whole plant and dividing it into several smaller plants. Some you can take a cutting from and plant in a pot, like a houseplant. Seed sowing is a good way to grow a lot of plants, as in the case of lavender for edging a border, for example. There is a huge selection of seeds from which to choose. Many new plants, as well as a few out-of-the-ordinary that might not be for sale as plants in Sweden, can often be had as seeds. It’s great fun to experiment with them, and it’s such an easy way to grow many different types of plants.
Perennials are sown in May and June, and the seedlings are potted up in their individual pots. They are not suitable for direct sowing in the ground or the flower bed because the plants will have trouble developing if they’re competing for resources with other plants. They’re very easily mistaken for weeds, and so are often weeded out. Seedlings are also a popular snack on the slug menu and can be eaten up. A lot of plants also take a long time to germinate, or need other special considerations.
How to’s—Sowing perennials
♦ Sow perennials in early summer in a flat box or in pots.
♦ When potting up, use individual pots and good planting soil.
♦ Leave the plants outside to grow, preferably under a roof to avoid hard rain that could damage the fragile plants.
♦ In the fall, move the plants to the greenhouse or to a cold frame.
♦ When the cold sets in, insulate the plants with fiber cloth or mats. Perennials are usually hardy to a few degrees below freezing, but are less forgiving of the chill when they are potted.
♦ When springtime light starts to shine, remove the fiber cloth cover and insulating materials from the plants to let them start growing. These are not forced to green up and flower early, like pansies and primroses—they are just given a more sheltered beginning.
♦ These plants can be placed directly outside before they are green, if you’re not in a hurry to see them flower, or if the greenhouse is too full. Otherwise, you can leave them inside.
♦ Water the plants regularly with added liquid fertilizer, same as with the outside bedding plants.
♦ When the new plants have grown bigger and have turned green, they can be set outside in a protected area, and then after a few weeks they can be transplanted to their allotted space in the flower bed.
♦ In summertime, the sturdy plants will flower a first time.
The later you sow in summer, the more critical it is to overwinter the pots in the greenhouse over the cold season. If you sow in May and June, the new plants can overwinter in a protected area under a roof or in a plastic tunnel. In early spring, the plants are moved outside if they are not to be forced into early flowering. At right: delphiniums/larkspurs are magnificent perennials that are easy to grow from seed.
First year flowering perennials
Many new perennials are bred to flower as soon as their first summer, if sown early enough. They will need to be sown in January and February, and pre-cultivated the same way as summer flowers (see chapter 5, page 40). If you’re in no terrible hurry, it’s better to sow in early summer and then wait for the first flowers the following year—the plants will be sturdier and the flowers more abundant. On the other hand, perennials that flower in their first year not only often flower later than their true time, but their growth is paltry and their flowers not much to look at.
Bushes and trees
Many trees and bushes are propagated through seed. Many are familiar with, for example, the horse chestnut, with its large brown seeds that are very easily sown. The common trait of many tree and bush seeds is that they take a significant amount of time to germinate—one to two years not being unusual. Some specimens, such as bush peonies and species roses, also need a cold spell in order to be able to germinate. Sown pots will stand in the greenhouse during the winter, where they’re protected from excess moisture, as well as hungry rodents and other animals. During summer, they stay outside and are covered by protective netting. Certain kinds of trees and bushes are propagated through cuttings, just like geraniums. These cuttings are usually taken in the summer (it will, however, depend on what type of plant it is) and they’re protected against dehydration in a mini greenhouse set within the greenhouse. More detailed and specialized literature is available to anyone further interested in propagation of plants.
Many herbs are perennials that can be sown like perennials, and then planted out the following year. You can oftentimes even plant them out the same year. The most important requirement for many of them is that the planting area be sunny and that the soil drain well—it needs to be loose and sandy, and to dry quickly, or else small plants risk rotting if they’re left standing in wet soil as winter sets in.
The most common herbs are propagated through seed. The striking red-leaved, variegated and white-speckled types of sage, mint and oregano often need to propagate through division, which is also the case with many kinds of thyme. The specimens listed below can all be grown from seed. They will grow a ‘true’ harvest and will produce fragrant and aromatic leaves. One exception is the type of tarragon used in cooking, which is French tarragon—it isn’t possible to sow it. The tarragon seed that you’ll find for sale is probably Russian tarragon, which is not nearly as delicate as its French counterpart; however, it usually only says ‘tarragon’ on the seed packet.
Perennials to grow from seed
Anise hyssop, Agastache foeniculum
Common hollyhock, Alcea rosea (biennial)
Greater masterwort, Astrantia major
Aubretia, Aubrieta x cultorum
Showy calamint, Calamintha grandiflora
Carpathian harebell—tussock bell flower, Campanula carpatica
Canterbury bells, Campanula medium (biennial, perennial)
Peach-leaved bellflower, Campanula persicifolia
Field larkspur, Delphinium
Sweet William, Dianthus barbatus (biennial, new annual plants now exist)
Maiden pink, Dianthus deltoides
Pinks, Dianthus plumarius
Purple foxglove, Digitalis purpurea
Purple coneflower, Echinacea purpurea
Blanket flower, Gaillardia x grandiflora
Sweet pea, Lathyrus latifolius
English lavender, Lavandula angustifolia
Shasta daisy, Leucanthemum x superbum
Oxeye daisy or marguerite, Leucanthemum vulgare
Garden lupin or large-leaved lupin, Lupinus polyphyllus
Russell lupin, Lupinus x regalis
Maltese cross—Scarlet lightning, Lychnis chalcedonica
Catnip, Nepeta x faassenii
Oriental poppy, Papaver orientale
Polyanthus primrose, Primula, Polyanthus group
English primrose, Primula vulgaris (former Primula aucaulis)
Self heal—Heal All, Prunella grandiflora
Woodland Sage, Salvia nemorosa
Painted Daisy, Tanacetum coccineum
Columbine meadow rue, Thalictrum aquilegiifolium
Meadow rue, Thalictrum delavayi
Tufted or horned violets, Viola, Cornuta group
Small lavender plant from seed.
Many lavender plants used for edging a ’Bella Rosa’ rose.
Herbs to grow from seed—Perennials
Garlic, Allium sativum (often planted as cloves)
Chives, Allium schoenaprasum
Garlic chives, chinese chives, Allium tuberosum (tastes like garlic)
Southern wormwood, Artemisia abratanum
Absinthe wormwood, Artemisia absinthium
Hyssop, Hyssopus officinalis (can give rose and white flowers but most usually blue)
English lavender, Lavandula angustifolia
Lovage, Levisticum officinale
Lemon balm, Melissa officinalis
Peppermint, Mentha x piperita
Sweet cicely, Myrrhis odorata
Russian oregano, Origanum vulgare
Garden parsley, Petroselinum crispum (biennial)
Rosemary, Rosmarinus officinalis (not hardy)
Common sage, Salvia officinalis
Common thyme, Thymus vulgare
Herbs to grow from seed—Annuals
Dill, Anethum graveolens
Garden chervil, Anthriscus cerefolium
Meridian fennel—Persian cumin, Carum carvi (can be perennial)
Cilantro, Coriandrum sativum
Fennel, Foeniculum vulgare
Sweet basil, Ocimum basilicum
Marjoram, Origanum majorana (can be perennial)
Aniseed, Pimpinella anisum