I have been growing brassicas from seed for a number of years and have always had a problem with them going leggy in their starting pots. They always have plenty of light. My Aunt, who lives in England, has the same problem with hers. Is there is a way to prevent the legginess?
You’re on the right track to get your seedlings off to their best start. Good, stocky transplants are vital to a good harvest from your plants.
There are two reasons that young seedlings become leggy – light or temperature or a combination of the two. The light could be too weak or too far away from the plant and the soil temperature or the air temperature could be too warm. Since brassicas are cool season plants it is generally best to start and grow the seedlings on the cool side. A soil temperature of about 55 degrees with daytime air temperatures of about 65 degrees and nighttime air temperatures of around 55 degrees is just about right. They need bright light and a large south window will work, but be sure to rotate them one-quarter turn everyday so they will be stocky.
Even better, since broccoli and other brassicas are often started earlier in the year than most seedlings and light levels are low, use a fluorescent light fixture with two 40 watt tubes and position them so that the lights are 2-inches above the starting medium. Keep raising the light as the plants grow so that the lights are always 2-inches above the leaf tips. Provide 14 – 16 hours of light each day.
Keep your seedling moist but not soggy, provide for good humidity and some fertilization at half strength about every two weeks. When they have developed the first set of true leaves, thin them to one seedling per pot or transplant them to individual pots. Continue growing (and rotating) them until it’s time to start hardening them off, about two weeks before planting time. I’ll bet you can already taste that first, sweet harvest.