For those gardeners who’ve gotten a jump on the planting season by starting seeds indoors, it’s usually safe to transplant cool-weather vegetables outdoors (including cabbages and other brassicas, beets, lettuces, spinach, and onions) by the middle of April. As soon as the soil can be worked, you can also direct sow vegetables that actually prefer cool weather (peas, lettuces, carrots, beets, chard, radishes, and turnips). Note: Warm-weather vegetable seedlings shouldn’t be transplanted until after Mother's Day, when the soil has warmed.
One very important step is needed before transplanting your seedlings outdoors... hardening off. The artificial light you’ve provided indoors is a pale substitute for the sun. If you’ve ever traveled to Florida in the middle of the winter, you know how easy it is to get sunburned. It’s the same with tender indoor-raised seedlings, if planted directly outside without hardening off first. The process of hardening off toughens up the young plants in two ways, both of which will prevent sunscald.
The first is to change the leaf cell structure of your tender seedlings. The chloroplasts in plant cells actually align differently to adjust to outdoor sunlight. The second is to acclimate the seedlings to wind and temperature variations. Together, these two factors of the hardening off process foster plant resilience.
Since these two changes take time, it’s important to expose your seedlings to outdoor
conditions gradually. As soon as daytime temperatures are in the low 40s, simply set the trays or pots of seedlings outside in full sun, at first for just an hour or 90 minutes. Each day forward, increase the time outdoors in full sun by about an hour. That means that in less than a week, you can start leaving the seedlings outside all day. Be sure to bring them back inside at night, unless the predicted overnight low stays well above freezing.
After a few weeks, you’ll notice deepening leaf color, stronger stems, and more rapid growth which are signs of acclimated plants that are ready to transplant successfully to the garden.
Happy gardening!