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Lanky cleome seedlings

Q: My cleome seedlings are leggy. Can they be buried with their cotyledons below soil level? Can this practice be used with most, if not all, flowering annuals or with just a select few?

Joseph Giordano, Flint, MI

Certain annuals grow leggy when started indoors. Photo/Illustration: Melissa Lucas

A: John C. Fech, a horticulture educator at the University of Nebraska-Lincoln Extension in Omaha, Nebraska, replies: Sure, you can try burying cleome seedlings with their cotyledons (first set of true leaves) below soil level, but there are more negatives than positives to this practice. The upside is that the stems may develop roots that will stabilize the plants. I recommend planting tomato seedlings this way as their stems are loaded with adventitious buds, which quickly form roots that increase stability and absorption capacity. Other than stoloniferous plants, I don’t know of any other particular genus or plant group that might benefit from this treatment.

Often the stems of warm-season annuals such as cosmos and cleome get stretched when started indoors. This is largely due to etiolated (stretched or lanky) growth that develops when starting seedlings under low light. Also, these annuals and others—such as annual larkspur, four o’clocks, and dill—have taproots that don’t respond well to having their roots disturbed. Here’s the downside: Placing the roots deeper than normal in soil can reduce oxygen availability and lead to root rot.

Rather than messing around with seedlings, I recommend direct sowing the seeds in fall or spring, depending on where you live. The seedlings will germinate at the right time, and the resulting stems will be sturdy. These annuals generally perform well in full sun. Thin seedlings after they reach a height of 4 inches or so to allow adequate light to reach all of the leaves. Eventually, space plants 8 to 15 inches apart.

If you want to scratch the green-thumb itch by starting seeds indoors in February, be sure to provide lots and lots of indoor light. You might be able to get by with fluorescent lights but probably not; they’ll most likely produce plants with spindly stems. Heavy-duty fixtures with 400- to 1,000-watt bulbs will provide the necessary input: at least 2,000-foot candles of light. In summer, a bright, sunny day produces about 10,000-foot candles.

From Fine Gardening 110, pp. 18