As your starts begin maturing, bearing their “true” leaves, we know the time is drawing near to transplant. Transplanting can be stressful for growers because it is common to lose plants to “transplant shock.” When our plants spend the first few weeks of their lives indoors, in a virtually weatherless environment, and then are dropped into the ground, they can become "shocked" due to the outside elements—and sometimes they are unable to recover. The heat and cold, sunlight, wind, and rain can all damage your plants when they aren’t used to receive so much of it. That is why you must gradually expose your plants to the outside elements on a daily basis. This process is referred to as “hardening off”.
The idea of “hardening off” is to set your plants outside, preferably on a mild overcast day or in the shade, for a set amount of time each day. Some gardeners recommend setting your plants out for an hour on the first day, two hours on the second day, and so on until it is time to transplant; however, we believe more care needs to be taken for the first few days. On the first day, we recommend letting your plants rest outside (still in their starter plugs or pots) for about thirty minutes to AT MOST an hour, checking on them every fifteen minutes. Upon any signs of wilting, burning, or shriveling, bring them back inside. We also recommend watering them substantially before placing them outdoors.
Knowing the hardiness of your plants will help you know when to set them outside, according to the conditions, and for how long. The hardiness level of a plant is its ability to withstand harsh conditions. Vegetables like broccoli, cabbage, and onions are very hardy and can withstand cold temperatures as low as 35 degrees Fahrenheit. So, that lets you know that these plants can hold their own fairly well in the cold and should be exposed gradually to warmer temperatures. A lot of gardeners and farmers actually start brassicas like broccoli and cabbage outdoors this month since they are such tenacious seeds, able to germinate and sprout during the fading cold nights. For the most part, we tend leave the indoor starting to the more tender (aka less hardy) plants.
Tender vegetable plants include tomatoes, peppers, eggplants, watermelons, and summer squash. These plants are more susceptible to climate elements because of their high fruit-bearing function. These are the kinds of garden plants that are predominantly started indoors during late February and throughout March. When setting them outside, keep a close eye on them since just a bit too much heat or cold can kill the young plants. Using a grow cloth, a greenhouse dome, or a grow tray turned upside-down to shield plants from the wind, rain, and sunlight is recommended for the first few days. “Hardening off” usually takes 7 to 10 days generally, but with low-hardiness plants like tomatoes, it can take up to fourteen days.
Don’t forget about flowers! Same rules apply for flowers. Some flowers are less hardy than others and require closer attention just like tender garden plants. Below is a list of seeds with the LOWEST temperatures they can withstand. If you’ve been following along, you’ll notice that they are the same list of garden vegetable and flower seeds we mentioned in our February newsletter about sowing indoors.
I used to agonize over how long to leave my starts out after a couple of days rest after transplanting them to 4" pots so they can build good root balls. Every year I lost numerous starts to scald, damping off and wind damage in spite of my best efforts. 3 years ago I came up with a brilliant idea to protect them while hardening…I cut the front of our cat litter jugs out leaving a lip to hold the pots in and put them out with the cutout facing away from the sun. This works best with the translucent style jugs so the light is diffuse but the plants are still protected from scald and they’re totally protected from the wind. The handles make them so easy to transport to the garden when they’re ready to plant as well.
Thank you! The details you offer on hardening off (especially lowest temperature) are very helpful! We’ve had to be very careful to protect our seedlings from bunnies as we do the hardening off, as the bunnies will come right up to our doorstep to eat seedlings if they are unprotected-just to warn folks.
How long do I “Hardening off” of broccoli, cabbage, and cauliflower ?