Greenhouse Vegetable Gardening (2015)

All flowers were grown in the greenhouse. Amaranth is in the middle, surrounded by light red scarlet monkey flower, yellow and red snapdragon, pink dianthus and yellow marigold in one wonderful mix.

Summer blooms—or flowers—is used as an all-purpose term to cover lots of different plants—usually annuals, but it can also include perennials, bulbs, corms, tubers and rhizomes (collectively known as geophytes), and semi-shrubs, all of which embellish the garden in the summer. Climate dictates where and how the flowers will grow and where they should be planted. Most summer blooms do not survive cold spells and snaps; this means that the flowering stops at the first sign of frost. The advantage of starting plants from seed indoors is that you can enjoy earlier flowers and a longer flowering period.

Some summer blooms, like Heliotrope, can be challenging, as they require long pre-planting preparation; others, like calendula are easily started. Plants, when started indoors and then planted in the garden, are usually referred to as transplants. Summer blooms can be transplants as well as direct-sown flowers, such as the cornflower. There is a heightened interest in growing summer blooms among hobby gardeners, resulting in more and more seeds and varieties becoming available for sale. Furthermore, shopping for seeds is easy and fast over the Internet, since they travel well and weigh very little. Don’t get taken in by the hype surrounding expensive newcomers, however—seed catalogues do tend to exaggerate their virtues.

Separate groups

Summer blooms can be divided into separate groups: flowers that propagate through vegetative reproduction using cuttings, corms, rhizomes, and bulbs, or through division; those that can propagate through both seeds and cuttings; and finally, those that can only be propagated through seeds.

Apart from differences in propagation, there are separate kinds of seedling growth. Many traditional transplanted plants require longer pre-cultivation periods. Petunia, annual phlox, dianthus, and different varieties of salvia like to take their time, while Marigolds grow much quicker. Thanks to a seedling start in the greenhouse, you can coax early blooms from more common varieties to make a nice addition to flower beds, which can then be planned to make a beautiful early flowering ensemble. You can also try unusual plants and create unique combinations. Indoor seed starting produces many plants from just one seed packet, so you’ll have the opportunity to decorate the garden lavishly, to make lush container plantings and flower beds where it would otherwise be challenging to get something to grow. It’s a really affordable pleasure too because you get so many flowers on even a small budget.

A store-bought marguerite shares space with nicotiana ‘tinkerbell’ started from seed, and petunia.

Early spring

There usually isn’t a lot of heat or light in a hobby greenhouse, which means that some plants simply can’t be grown in that environment. You can select some plants to start growing from seed, while acquiring others from garden centers and nurseries. There is a large color palette to choose from if you decide to grow snapdragon, scarlet monkey flower, and trumpet flower. Once you’ve gained some experience, you can try raising more demanding plants. Growing a large and robust crop of petunias, for example, is not exactly child’s play and will meet your challenge.

The most difficult part of seed starting is to know when to begin. Timing depends on when the plants are to be transplanted, so it’s necessary to count backwards. In the south of Sweden, you can transplant out as early as the end of April, while in the north you have to wait until midsummer.

The light exerts a strong influence, too. If the seeds are sown in February, it could take as long as fifteen weeks for Petunia seedlings to grow large enough for transplanting; if sown in April they may only take eight weeks. The later in the season the sowing is done, the lighter and warmer the environment will be, thus the faster the seeds will grow.

How large the plants typically grow before transplanting affects your sowing date. It is best if plants have grown a bit but have not yet reached the flowering stage before they are planted out. Totally green plants take root quicker and adjust faster to their new environment. A flower in bloom, on the other hand, might stop growing because of the shock of being moved outside from the greenhouse. This means that it can stop flowering, during which time the green plant can catch up and flower earlier. When buying plants, you’ll want to see flowers so you know what color and type of flower to expect. If you start seeds yourself, however, you’ll already know what you’ve got so the plants don’t have to be in bloom.

How is it done?

Seed starting and potting up follow the same steps, whether it be flowers or vegetables. The differences lie mostly in growth times and the size of pots to use when potting up. Some plants also need pinching to grow sturdy, while pinching is hardly ever necessary with vegetables. All plants follow, on the whole, the order described here. Further details about soil, water, and fertilizing can be found in the section about seed starting and planting, on page 29. There you’ll also find a detailed description on how to sow seeds and how to pot up into bigger containers.

The best way to start the seeds is inside your house. With few exceptions, most seeds need warmth to grow. A room temperature of 20°C to 25°C (68°F to 77°F) works well; put the seed tray in a window that gets good light exposure, and it won’t need to be moved into the greenhouse until there is green showing. In some cases this means the seed tray will stay in the house for two to four weeks before it needs to be moved out. The date at which you can start the seeds depends on when you can transfer them to the greenhouse, which in turn depends on the weather. If you have a fan heater, you might be able to move the seedlings into the greenhouse earlier.

Some seeds, like impatiens, summer geranium, and snapdragon, require light to germinate, so their seeds cannot be covered with soil. To prevent the soil surface from drying out, you’ll need to cover the seeds with plastic or glass, or use a mini greenhouse.

Then there are seeds that need stratification through chilling before they can germinate—nature has placed barriers in the seed to prevent it from germinating at the wrong time of the year. Some seeds require total darkness; yet others must bask in at least twelve hours of light per day. Reputable seed sellers have information about special requirements printed on the seed packets. Most plants, however, aren’t complicated and can be sown following the instructions above.

What to choose and what to steer clear of

One of winter’s great pleasures is to browse through seed catalogs. If you order seeds well in advance they should arrive by January or February. Once you’ve decided which plants you’d like to start from seed, you’ll need to figure out the right time to start each type of seed. A good way to do this is to stagger the sowing of different seeds in order to avoid having to care for everything all at once. (The seeds themselves don’t take up much space, but the seedlings will eventually need to be potted up and then space might become an issue.) With this in mind, sort the seed packets into three groups:

♦ Plants that need a long period of time indoors with lots of light in order to bloom—twelve to sixteen weeks not being unusual.

♦ Plants that need less time—about six to ten weeks—and where success is more likely.

♦ Plants that can be directly sown, but which will offer up better results and earlier blooms if started inside.

Some plants need to be propagated through cuttings in order to be able to grow true. True versions of fuchsia and geraniums, as well as many wave petunias, blue potato bush, daisies and angels trumpet can be bought as small or big plants.

Table of summer flowers

The following pages can help you plan your garden, and can be used as a guide to plants that can be started from seed indoors. In these lists you’ll find the name and the botanical name in alphabetical order, in addition to the amount of time it takes each plant to germinate.

Suitable germination temperatures will be noted on the seed packet—it’s usually around 20°C to 25°C (68°F to 77°F). You can start the seeds anytime in the spring, depending when you’d like to see them bloom. Most plant seeds are sown in March and April, with a few earlier in January and February.

Growing time, as stated in the list, is from the time the seed is sown until the plant is ready for out-planting. This doesn’t necessarily mean the plant is in flower—that can take several months depending on the plant variety. The guideline is not to be followed too strictly: it’s sometimes acceptable to plant out both earlier or later than the listed time—it will depend on weather conditions.

The preferred growing temperature is noted, too, but again it’s occasionally open to variation. Most plants thrive in 15°C to 18°C (59°F to 64°F). The cooler and darker the plants’ environment, the longer it will take them to grow. It doesn’t matter if the plants are on the small side at first—they’ll quickly catch up once they’re transplanted outside.

Mini greenhouses—that is, plastic boxes with transparent covers—are excellent for sowing and pre-cultivation indoors. Early seed starting in the kitchen.

Indoor seed starting and pre-cultivation for summer flowers

Plants with an asterisk (*) beside their name can be direct-sown in a flower bed and will still have time to flower—in the south of Sweden at least.

Common snapdragon is a low-growing variety that is easy to start from seed and flowers quite early.

Sweet peas need a lot of time to germinate. Soak the seeds in water overnight, sow and start from seed to enjoy early flowers. This variety is called ‘Blue Ripple’.

Black-eyed susan flowers late in summer if direct-seeded. Pre-cultivation in-house will make them bloom earlier. Every fall seeds are collected from this old-timer, and are set aside for spring sowing.

Abundant flowering beauty on a sunny step. The mini-petunia ‘million bells’ is propagated through cuttings.

Summer flowers for different sites

You can try many different and tempting new plants by starting seeds indoors, but you must still take into consideration the outdoor site where the plant is going to end up growing. Summer flowers don’t all thrive in similar locations—most of the richly flowering plants need abundant sunshine, plenty of soil, water, and nutrients, while others will tolerate tougher conditions.

It can get hot, dry and perhaps windy for window and balcony boxes on south-facing decks or steps. There you’ll have to put plants that can stand up to searing heat over several hours; geraniums and twinspur are examples of hardy blooms that can weather these kinds of conditions. Cut-leaved daisy, yellow cosmos, creeping zinnia, and licorice plant are also surprisingly tough and large, and won’t be as colorful and lush with flowers if placed in the shade. Fuchsia, ornamental bacopa, impatiens, and geraniums, on the other hand, make excellent shade plants. Nicotiana, New Guinea impatiens and most leafy plants also do well in the shade.

Shuffle and Deal

Seed starting, aside from allowing you to try a lot of plants that are not sold as seedlings, offers you the opportunity to put together your own flower compositions. You can use different companion plants earlier in the season in big containers to decorate your garden, before it’s time to plant the flowers in the ground. Don’t crowd too many plants in each pot, however, as they’ll need space to grow and fully develop over the summer. Also, leave larger containers in the greenhouse warmth a little longer—the plants will grow together into an attractive entity and they’ll be more uniform in size. If springtime warms up quickly, the plant’s growth will explode and will go from green seedling to a flowering plant in as little as a week’s time. Place the pots outside to fully enjoy those beautiful blooms.

Take pictures of the plantings, and write down the names of the plants to have on hand as reference for future plantings. Take note of the good plants as well as the lesser performing ones. The fun lies in being able to mix flower color, leaf color as well as cultivation method. Many leafy plants are used as fillers to add calm to companion-plant flowers.

Hanging planters

The advantage of starting your own plants is that you will enjoy an abundance of flowers. A clustering of many small plants has a more beautiful visual impact than only a few big ones in a container; this is especially true for hanging planters.

Small plants are used to make English-style hanging basket planters. The planter, made of coarse netting, is lined with plastic and then moss. Plant roots are introduced from the outside through the netting holes. The planter is filled with soil bit by bit, while the sides are planted in rows. At the very top, once the planter’s upper rim is reached, a sturdier and upright plant is planted, such as a tuberous begonia or summer dahlia. By all means mix several different types of plants to highlight their beauty—lobelia, creeping charlie (ground-ivy), licorice plant, impatiens, ornamental bacopa, yellow cosmos, cut-leaved daisy, mini-petunia and ivy, for example. Hanging planters require nutrient-rich, quality soil and regular watering with fertilizer each day over the summer. Many plants in relatively small baskets need a lot of nutrients and water to be able to continue flowering and growing.

To develop their full effect, hanging baskets can be planted three to four weeks before being placed outside. Use plants that have been cultivated in the greenhouse, and transplant them into hanging baskets in the spring. Leave the hanging basket in the greenhouse to let the plants branch out fully into one big plant. Use large hanging baskets, preferably holding 5 to 10 liters (1 1/4 to 2 1/2 gallons) of soil. The larger the hanging basket, the less likely it is to dry out.

An early, store-bought twinspur stays in the greenhouse until all risk of nighttime frost has passed.

Flowering plants for hanging baskets

Mexican paintbrush, Ageratum houstonianum

Wax begonia, Begonia x semperflorens-cultorum

Tuberous begonia, Begonia x tuberhybrida

Fern-leaved beggarticks, Bidens ferulifolia

Cut-leaved daisy, Brachyscome

Million bells, Calibrachoa

Calitunia, x Calitunia

Star of Bethlehem, Campanula isophylla

Blue rock bindweed, Convolvulus sabatius

Carnation, Dianthus chariophyllis

China pink, Dianthus chinensis

Cottage pink, Dianthus xsemperflorens

Twinspur, Diascia

Blue daisy, Felicia heterophylla

Hybrid fuchsia, Fuchsia x hybrida

African daisy, treasure flower, Gazania

Garden vervain, Glandularia (Verbena) x hybrida

New Guinea impatiens, Impatiens hawkeri

Busy Lizzy impatiens, Impatiens walleriana

Lobelia, edging, hanging baskets, Lobelia erinus

Sweet alyssum, Lobularia maritima

Nemesia, Cape Jewels, Nemesia strumosa

Ivy-Leaf geranium, cascading geranium, Pelargonium peltatum

Spreading petunia, wave, Petunia x hybrida

Moss rose purslane, moss rose, Portulaca grandiflora

Creeping zinnia, Sanvitalia procumbens

Black-eyed Susan vine, Thunbergia alata

Garden nasturnium, Tropaeolum majus

Canary creeper, Tropaeolum peregrinum

Miniature pansy, tricolor, Viola

Leafy plants for hanging baskets

Bugleweed, Blue bugle, Ajuga reptans

English ivy, European ivy, Hedera helix

Curry plant, Helichrysum italicum

Licorice plant, Helichysum petiolare

Creeping lamium, Deadnettle, Lamium

Parrot’s beak, Lotus berthelotii

Creeping Jenny, Moneywort, Lysimacchia nummularia

Parsley, Petroselinum crispum

Spurflower, a variety, Plectranthus

Cherry tomatoes, Solanum lycopersicum

Coleus, Solenostemon scutellarioides

The larger the hanging basket, the prettier the blooms.

Long-blooming hanging baskets

Large, beautiful summer blooms such as cascading geraniums and wave petunias have become very popular. These sturdy plants are usually propagated through cuttings, which means you will have to buy the plants. (Some species can be cultivated from seed but it is better to buy the plants, as it takes a very long time to cultivate them.) They’re sold in hanging baskets, but if they’re transplanted into larger baskets they will grow even more impressive. There are special hanging baskets outfitted with large water containers that are made specifically for these huge plants. It’s necessary to have large spreading wave petunias and cascading geraniums to hide the water containers, but then again these plants do require a very large amount of water and fertilizer.

Towards the end of the summer, when the plant is at its fullest, you can keep it flowering and prevent it from wilting—even if you leave home for the weekend. Typically, big plants in pots dry out quickly at the end of summer, and a weekend of neglect is enough for them to collapse. The plant is not as young and spry in the fall and cannot survive drying out. Many summer flowers that wilt and die down in August have already started to dry up or are suffering from a lack of nutrients. If you make sure they’re placed in large pots and are given fertilized water regularly, they will keep flowering without any problems until first frost. Some plants, like ornamental bacopa, twinspur, and licorice plant can even survive one or two nighttime cold snaps.

Purchasing plants

Owning a greenhouse allows one to be picky. You can buy plants for planting out early in the spring, and cultivate them in the greenhouse until it’s time to transplant them outside. Spring is when you’ll find the largest selection of plants at garden centers and nurseries; it’s also the ideal time to discover thrilling newcomers. It’s better to hold off on transplanting until the weather warms up and the risk of an overnight frost is over, because if you plant out too early, the plants will stop growing and their leaves will turn yellow. It’s preferable to keep them in the greenhouse, ventilated on sunny days and sheltered from frost at night, as this will acclimatize the plants somewhat to their future life outside. If they’re planted in pots and hanging baskets they will have plenty of time to grow, and thus will be hardy and healthy at transplanting time. That way you also won’t have the worry about flowers and long branches getting damaged on the trip home from the garden center or nursery.

Exotic colors

Lots of vibrantly colored plants with exotic allure originate in warmer countries. Most of them aren’t perennials in Sweden, yet they’ve been an integral part of our gardening scene for many years. The dahlia is an example of such a plant. It requires a substantial amount of work—you can save its tubers from year to year, but they need to be planted in good soil, dug up in the fall, and then kept in a frost-free but cool environment. Despite this doting care, there’s no guarantee that they’ll bloom before first frost.

Peruvian lily, also called parrot flower and its roots; a rewarding geophyte plant overwintered like a dahlia.

Seed-started annual summer dahlia ‘fireworks.’

You’ll save a lot of time if you plant the roots in large pots and let them start in the greenhouse. Dahlias usually need warm soil, which means that you cannot plant them in the central part of Sweden until the month of June; if the frost arrives by the end of August that doesn’t leave you much time to enjoy the flowers. If they are started in the greenhouse, however, they can be transplanted as soon as the risk of night frost is over.

It is practical to sow and plant in the greenhouse as it doesn’t matter if some soil ends up on the floor.

Many geophytes

There are many rewarding plant bulbs that will flower beautifully if they are started early. Peruvian lily, fresia, canna, and begonia all need to start inside if they are to reach full bloom. Peacock lily and gladiolii can be planted out directly, but are also good to pre-cultivate inside to achieve earlier flowers, just like anemone and persian buttercup. Uncertain hardiness in red hot poker/torch lilies and montbretia (which are perennials in the south of Sweden) make them good candidates to be started early and overwintered inside.

Gentian sage, which gets fat white roots, as well as four o’clocks with their carrot-like roots, are, surprisingly, geophytes. They are used as annuals but are in fact perennials. They can have a long blooming period if their roots are dug up, kept free of frost, and then planted in a pot in the greenhouse early in spring.

Sage grows increasingly large year after year, and you can divide it. Four o’clocks grow into sturdy plants similar to low-growing bushes, with an abundance of small colorful flowers.

How to’s—Planting and care of geophytes

In spring, geophytes, i.e., bulbs and roots, are planted in large pots to give them plenty of space to grow. Plant them directly in attractive planters, or in large plastic pots with drainage holes. Plastic buckets with drainage holes drilled at the bottom also work perfectly well.

♦ Place a layer of nutrient-rich planting soil in the bottom of the pot or container. The depth of the layer depends on the plant; dahlias require approximately 20 cm (7 3/4”).

♦ Spread out the roots or position the geophytes on the surface. Press down so there’s proper contact with the soil.

♦ Cover with planting soil according to the instructions on the soil bag. Dahlia roots have to be covered, and the tips (root cap) should be about 5 cm (2”) below the surface.

♦ Bulbs such as gladiolus can have more soil added little by little as they grow. Canna lilies require plenty of growing space and nutrient-rich soil.

♦ Water generously at planting time, and then more sparingly afterwards until the plants start to show green shoots.

♦ Leave the plants in a sunny spot in the greenhouse to give them time to fill in. Once the weather allows, they can be placed outside.

♦ Bury the entire pot in the flower bed, or place the decorative planter in its chosen spot.

♦ Another way is to ease the plant out of the pot carefully, and plant it directly in the ground. Place the pot on its side and pull it off gently to avoid damaging the fragile plant stems.

♦ Plant the clumps—complete with stems and leaves—at approximately the same depth as in the pot. Root tips should be slightly more than an inch below the soil surface.

♦ Water and fertilize generously during the summer. Most bulb plants need a large amount of added nutrition to grow roots and bulbs in the following years.

♦ Bulbs and other geophytes are dug up in the fall, to be stored over winter according to their specific needs. If the plant has been potted, then the whole pot and plant are overwintered together, and the plant is repotted into new, nutrient-rich soil the following spring. Another advantage of container growing is that the whole plant can be moved inside the greenhouse if there is risk of frost. As the buds open into flowers, these can then be used as cut flowers.

Hardy lilies

Lilies are different in that many are hardy and can overwinter in the ground as long as the soil is well drained; however the large, fleshy bulbs are prone to rotting. If bought in early spring and planted in pots in the greenhouse, they will have time to get started and will produce earlier blooms. Lilies need to be planted deep in the soil, but there is a way to accelerate their growth:

♦ Plant the lily bulb in a bucket or a deep pot with a 10 cm (4”) layer of soil at the bottom. Fill with soil until the bulb is covered.

♦ When the plant shows green shoots, add more soil several times as the plant grows, until the pot or bucket is filled to the brim.

♦ Once the stalk has grown above the soil surface, it can be planted carefully in the ground in its designated place.

Another method is to keep lilies potted through the whole season; in this case the pots and bulbs will need to be protected from harsh winter weather. Potted lilies are considered more or less a common ‘house plant,’ and are sold for decoration both inside the home and outside. Many lilies can be transplanted into the garden, and if they are hardy they will return year after year.

Lilies are very tall and slender, and therefore perfectly suited to be planted between other plants. If lilies are started early in pots, they can be placed outside, their flowers filling empty spots in flower beds.

China (Annual) aster. Below: oriental poppy.