Starting Seeds in the Greenhouse: Aster/Composite Family Vegetables


Lettuces are an excellent choice for growing in cold frames and unheated greenhouses, provided they don’t freeze, of course. But have you ever wondered what other plants might be closely related to this commonly grown leafy vegetable? The Aster or Composite family is a big one—so big, in fact, that it contains weeds and ornamental flowers along with useful vegetables. You’ve probably grown a few of these plants in your garden, but others may have been too exotic or intimidating to try in the past. Read on for the secrets to success when starting the seeds of the diverse vegetable members of the Aster family, which includes lettuce, artichokes and sunflowers.

What’s in a name?

The Aster or Composite family is one of the biggest in the plant world. The over 23,000 distinct species may not appear related at first, but they share some very distinct common traits. The Aster family has a unique and special flower, the most recognizable form in the world—a daisy-like flower. It seems like such an innocuous little flower, but actually has a complex design: what looks like a single flower is actually a flower head composed of many tiny, star-shaped flowers surrounded by petals.

Although the plants in the Aster family don’t look much alike on the surface, they share many specific metabolic processes. The seeds of these plants are also similar, with each tiny flower growing into only one seed as part of a larger seed head. We don’t always see this part of the process because we eat many of the flowers, leaves or taproots before the plants are ever able to reproduce, but if you let your lettuces, artichokes or salsify go to seed, the resemblance is striking.

Introducing the Asters

Members of the Aster family are fairly easy to grow, though many take a while to reach maturity. Unless you have a long growing season, plan to grow your Aster family members in containers in your greenhouse. A few members can be grown as perennials in limited areas, where they make striking edible landscaping.

Cardoon  You may have never heard of cardoons, but they’re dramatic plants that reach harvest in just 60 days and can also winter over in warm areas. Related to the globe artichoke, this thistle-like plant’s roots and leaves can be eaten. Cardoon seeds should be started in individual biodegradable pots just 1/4 inch below the surface of the soil. Maintain soil temperatures between 65 and 75 degrees Fahrenheit (18 to 23 degrees Celsius) and seedlings will emerge in 10 to 20 days. Plant at least 30 percent more than you expect to need, since germination rates are often about 70 percent with another 20 percent of seedlings at risk for slow development or albinism.

Otherwise, cardoons are generally hardy and can be transplanted six to eight weeks from emergence. Transplant them to the garden after frost risk has passed, but when a window of about 10 days of temperatures below 50 degrees Fahrenheit (10 degrees Celsius) remain to induce earlier budding. These plants can be grown as perennials in USDA Plant Hardiness Zones 7 and 8.

Chicory – More than just a pretty blue flower, chicory has been a popular source of leafy greens for centuries. Even though the leaves are bitter, they are commonly used in mixed green salads. Chicory are beautiful cottage garden plants when allowed to go to seed; their leaves can be harvested in as few as three to five weeks from emergence.

Germinate chicory seeds in about a week at temperatures ranging from 45 to 85 degrees Fahrenheit (7 to 29 degrees Celsius). Direct sow chicory in the fall or grow it in gallon pots in the greenhouse, just barely covered with soil. You can transplant very young chicory plants, but some species have long tap roots, so start them in biodegradable pots.

Endive – Yet another popular salad green, endive and its flat-leaved sibling escarole are easy to start in a cool spot—short days and cool nights are ideal for this bitter leaf. It only needs about three months from seed to harvest, but you can get a jump on a short growing season by planting in biodegradable pots six to eight weeks before the last frost in your area.

Seeds will germinate at temperatures from 35 to 85 degrees Fahrenheit (2 to 29 degrees Celsius), though 70 to 75 degrees Fahrenheit (21 to 24 degrees Celsius) is ideal. Plant your seeds 1/4 inch below the surface of the soil whether seeding in the garden or in pots and watch for them to emerge in 5 to 7 days. It may take a little longer if temperatures are really low.

Globe Artichoke – Among the giants of the Aster vegetables, globe artichokes make beautiful perennial plantings in USDA Plant Hardiness Zones 6 through 9, but can be grown as annuals elsewhere if they’re started in the greenhouse eight to 12 weeks before the last frost. Globe artichokes can reproduce by root divisions or seeds, but seeds are often easier to find.

Start seeds in biodegradable pots at temperatures between 70 and 80 degrees Fahrenheit (21 to 27 degrees Celsius) and expect emergence in eight to 12 days. Young starts can be cold treated to induce first-year flowering if you expose them to temperatures between 35 and 50 degrees Fahrenheit (2 to 10 degrees Celsius) for about 10 days before transplanting.

Lettuce – Appearing in a wide range of sizes, colors and shapes, there’s a lettuce perfect for everybody, no matter where they live. Head lettuces are difficult for the home grower to properly produce, but leaf lettuces are excellent for growing indoors and out. They also germinate quickly and some varieties are ready for harvest in as few as 45 days.

Lettuce seeds are small, but not impossibly tiny—still, it may help to mix them with some coarse sand to get better distribution if you’re growing your lettuce in trays for microgreens. Otherwise, sow seeds for transplants no more than 1/8 inch deep in individual cells three to four weeks before you plant to set them in the garden. They’ll germinate in as few as two days at 70 degrees Fahrenheit (21 degrees Celsius), but will emerge at any temperature between 45 and 85 degrees Fahrenheit (7 to 29 degrees Celsius) in up to about two weeks.

Radicchio – The crunchy, red leafy heads are a form of chicory and can be germinated in much the same way. Because they can tolerate a great deal of cold weather, many growers direct seed them to avoid the risk of transplanting stress that can cause bolting. If you want to start yours indoors, use biodegradable pots and replant them, pot and all, to minimize shock.

Radicchio seeds will germinate at temperatures from 45 to 85 degrees Fahrenheit (7 to 29 degrees Celsius), though 70 to 75 degrees Fahrenheit (21 to 24 degrees Celsius) is ideal. Expect emergence in seven to 10 days when seeds are planted no more than 1/2 inch deep.

Salsify and Scorzonera – These unusual root veggies were once American favorites, but are now much less common. Salsify is said to have an oyster-like flavor and the black-skinned, white flesh of scorzonera is unmistakable.  These root veggies don’t transplant well, but can be grown in pots like carrots.

Sow these seeds one to three inches apart, 1/2 inch below the soil surface. You’ll see good germination at temperatures between 55 and 75 degrees Fahrenheit (13 to 24 degrees Celsius), but germination can take up to three weeks. These edible roots need a long growing season, sometimes as much as 120 days, so plan accordingly.

Sunflower – An iconic flower of the American prairie, the sunflower has been hybridized into dozens of shapes, sizes and colors to meet the needs of growers everywhere. These popular plants are easy to grow and germinate, provided you leave enough space for the particular variety in question. Some species shoot up quickly, so plant your seeds in three to four inch biodegradable pots if you’re planning on transplanting later.

Sunflower seeds germinate best between 68 and 86 degrees Fahrenheit (20 to 30 degrees Celsius), emerging in five to 10 days. Start them two to three weeks before the last frost if you’re going to transplant, otherwise wait until the soil has warmed and plant your seeds directly in the garden or pot where they’ll spend the season.

Kristi Waterworth
image: NAPARAZZI (Creative Commons BY-SA)
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