Greenhouse Vegetable Gardening (2015)

Cell tray growing produces many plants for planting out. Here are different kinds of lettuce.

Fresh and newly harvested vegetables are an integral part of a high quality of life. A greenhouse gives us the opportunity to start plants from seed, which are then easier to cultivate and to make more efficient use of the space in the greenhouse. It also helps curtail the need for extensive weeding, and promotes an earlier and more productive harvest.

The best way to avoid weeding is to start seeds, and then plant the seedlings outside when they’re ready. If you’ve let your designated plot get overgrown with weeds—perhaps more than once—just remove them all before setting out the vegetable plants. It can sometimes be hard to tell a weed from a seedling, and then to weed between the plants, but you can take guesswork out of it altogether if the plants are put into a freshly weeded plot.

Start in early spring by working up the vegetable garden. Weed seeds and mature weeds are already in the ground, and they usually turn green long before the cultivated plants do. Raking the soil thoroughly will bring more seeds to the surface where they then will germinate; they can be removed a week or so later. If the soil is dry, watering can coax more seeds to germinate. Once the weeds are green, walk with a long-handle or draw hoe over the whole surface and weed. Do this on a sunny day, and leave the weeds out to dry up until the next day. You’ll need to repeat this routine once or twice. A roto-tiller can also be used to clean the soil’s surface. Meanwhile, your plants will be waiting and growing inside the greenhouse.

Plant cell trays

Plant cell trays are convenient because the seedlings don’t need to be potted up after being sown. They get a proper start, and there’s no need to tear up and disturb their roots. The cells are placed directly in the vegetable plot and the young plants can continue to grow unimpeded. They also leave room for many more plants to grow than if each were in its own individual pot, and they take up fewer resources. The seeds germinate and grow more evenly in warmed soil in the greenhouse than in the ground. More seeds germinate and their growth is stronger because they get a quicker head start.

The main inconvenience of cell trays is that the plants can’t be left too long in the cells. There’s only room for a small amount of soil, so the young plants can quickly become root-bound if they’re not planted out in time. When this happens their growth comes to a stop, and they experience difficulty growing properly once they are planted out.

Beans need warm soil. They get this in the greenhouse when planted in cell trays with big holes.

To save space, corn is started in cell trays before being potted up.

Vegetables that can be started in cell trays

Onions, Allium cepa

Scallions, (Spring onions), Allium fistulosum

Leeks, Allium porrum

Dill, Anethum groveolens

Chard, Beta vulgaris

Cauliflower, Brassica

Broccoli, Brassica

Brussels sprouts, Brassica

Red cabbage, Brassica

Spring (Early) cabbage, Brassica

White cabbage, Brassica

Mizuna (Japanese mustard) cabbage, Brassica

arugula, Eruca

Lettuce, Lactuca

Parsley, Petroselinum crispum

Beans, Phaseolus vulgaris

Peas, Pisum sativum

Spinach, Spinacia oleracea

How to’s—Vegetable cultivation

♦ Vegetable plants can be started very simply from seed by using cell trays or flat trays. They look like large plastic trays with plenty of small perforations at the bottom. These small holes, also called cells, are filled with soil that is packed down firmly.

♦ Water the surface thoroughly.

♦ Plant a few seeds in each cell, and cover according to packet instructions.

♦ Keep the cell tray in a place at room temperature. It doesn’t need to be exposed to light until the surface shows sign of green; then it needs to be moved into the light immediately. Sowing is usually done in the house or in the greenhouse—the greenhouse being preferable, as filling the trays up with soil can be a messy job.

♦ Once the seeds have germinated, clip or pinch off plants in order to leave just one shoot in each cell.

♦ Let the seedlings grow for two to three weeks, depending on the type of plant. They’ll be ready to transplant once the roots are big enough to keep the whole cell—soil and all—together so that it can be moved in one piece.

♦ Once the seedlings have emerged from the soil in the cells, you’ll need to add weak fertilizer each time you water them, because planting soil contains no nutrients. The best way to do this is to place the cell tray into a plastic tray and add water directly into the plastic tray; this will let the soil draw up the water and fertilizer from underneath.

♦ Many cell trays come in a complete kit (root trainers) with cells, reservoir pan and holder for easy removal. If there isn’t a holder, use a small, blunt wooden pin instead.

♦ Never pull out or yank on the plant. Put the seed cell on its side, and use your fingers to gently nudge the plant out of the cell if it’s stuck.

Planting

When the plants reach a few centimeters (about an inch) high, it’s time to plant them out. They’ll need to be hardened off for a week or two before transplanting, so move them outdoors in the morning and then bring them back inside at night, during their last week in the greenhouse. When planting them outside, you can amend the soil around each plant by adding some long-acting fertilizer, if needed. A small indentation around each plant will help with irrigation—use a watering can without a spray nozzle, and gently aim water at this indentation around each plant. Water the plant even if the soil is already wet so it can close up around the root clump. The root clump should not be exposed above the soil surface, or it will dry out very quickly.

Seed directly in the cell. Above right: There are many kinds of cell trays. Middle: The cells are emptied out with the help of pop-out brims. Below: The roots and soil should hold together in the cell that is ready to be transplanted.

When all plants are in place and watered, cover them with a fiber cloth row cover (placed over metal hoops) to ensure an even better outcome. The cover emits warmth during the night, and protects the plants against strong sun during the day. The plants will acclimatize quicker to their new environment if the cover is left on, and it also protects them against attacks from cabbage moths and other flying insects—but only if it’s left on during the greater part of the summer. Alternative housing for the plants is in a cold frame (see chapter 10, page 92). The cells can also be planted in the ground inside the greenhouse to provide an extra early harvest of tender springtime vegetables.

Root vegetables

Carrots and beets—vegetables grown for their edible, tasty roots—can also be started in cell trays. In fact, they should not be started in normal pots, as their roots will be damaged when the small plants get potted up. However, the root in the cell can continue to grow undisturbed after transplanting—the trick is to pack the soil just right. The soil needs to stay together but can’t be packed so tightly that the roots can’t penetrate, so planting these vegetables takes a bit of practice. Here it’s critical to have only one plant in each cell. If you see several seedlings, use a pair of nail scissors to avoid damaging the roots, and carefully cut away all plants except one. It’s not worth your time and effort to start radishes from seed, as they are such fast growers.

Cell trays can accommodate many plants in a small area. Still, the greenhouse will fill up.

Herbs

Parsley, chives and dill are examples of herbs that are frequently used in Swedish food, but it can be tricky to get their seeds to germinate. Dill needs warmth and is very susceptible to root rot; it’s good to sow dill in cell trays and then transplant it outside in a warm and sheltered spot.

Parsley is often in competition with weeds, and it’s challenging to weed it since you can hardly see the row of parsley, as it’s a very slow grower. You can dodge this problem by sowing parsley in cell trays. Another benefit to starting herbs this way is when a sown plant must stand a while longer before it can be transplanted out: the smaller the space it takes up, the better.

Even perennial herbs can be sown in cell trays to facilitate planting out. Salvia, thyme and rosemary, for example, can be seed started and are not very demanding when it comes to soil and nutrients. They can stay in cell trays until they reach 4 to 5 cm (1.5” to 2.5”) and are big enough to be transplanted into a sunny herb garden. These plants are not hardy all over the country, so some may need to be sown anew each year.

Common garden pots

You can seed start vegetables in common garden pots. Many of the sturdier plants are not good candidates to be started in cell trays—plants such as melon, cucumber, pumpkin, squash, chili and bell peppers, cape gooseberry, artichoke, and chard (also called artichoke thistle) need larger pots and long pre-cultivation care. All plants of the genus cucumis (cucumbers) can be directly sown into large pots, while others can be sown and harvested the same way as different kinds of flower plants.

A profusion of summer flowers

For an abundance of summer flowers with which to make bouquets, cell trays work beautifully. Plants such as China aster, marigold, painted daisy, snapdragon and rose mallow are a great fit for seed starting in cell trays. In this way you can start up lots of flowers while only using a minimal amount of space. Transplant them into the vegetable patch once they’re big enough to be moved—they should have reached between 3 and 5 cm (1.5” to 2.5”) in height—as they’re sown and grown the same way as vegetables.

Many plants can be started in cell trays. From the top: celery, snapdragon and cress.

We have already been harvesting lettuce for a full month when the trees start to leaf out.