Fertilizer and water
- Do not fertilize or water your seedlings during hardening off.
Don’t worry if the leaves of your seedlings start to look a little yellow—this is normal, as the plants are adjusting to more sun and lower humidity. Seedlings are used to being in a sheltered environment where they get watered often, so they’re unlikely to withstand light rain or drizzle during this phase. If you fertilize or water them, they’ll be less prepared for the outdoors, so don’t do it!
Light
A gradual increase in light is key to the hardening off process. If you’re keeping your seedlings indoors, place them in a south-facing window for one hour per day on the first day. Leave them there for another hour and a half on the second day, two hours on the third and so forth. On each subsequent day, add an additional half-hour until you’ve reached a full six hours of direct sunlight exposure.
The increased light helps to prevent legginess, water loss, stress and transplant shock before your seedlings ever move outside. Seedlings that are accustomed to this level of light will be able to thrive once they’re planted in their final location.
Soil and temperature
“There’s a difference between temperatures that will have you jumping for joy and ones that kill your plants.”
The best time to transplant seedlings is when the weather is favorable, such as in late winter or early spring. If you live in a place with long, cold winters, you need to be especially careful not to let the soil get too warm. A temperature of about 40ºF is ideal for seeds’ germination, but it isn’t enough to ensure their survival once they’re out of their pots and into the ground. Plants need temperatures above freezing (32ºF) during early spring and into summer so they can become established before their roots are killed by frost or frozen by cold weather. And remember: if your seedlings aren’t ready to be transplanted outdoors yet, they should stay inside until they are—they’ll still be fine indoors at lower temperatures than those needed for outdoor planting.
Wind
Wind is another factor to consider. Most plants, especially seedlings and transplants that are just getting established, do not like wind. In fact, some plants can’t tolerate wind at all. Wind dries out newly planted seedlings and can prevent them from developing a good root system. Also, wind can carry pests and diseases into your garden that may not have been there already. Make sure you set up your tender seedlings in a spot sheltered from the wind or else they won’t make it through hardening off!
The number one way to harden off your seedlings is to take it slowly.
The number one way to harden off your seedlings is to take it slowly. Seedlings have been sheltered in the warmth and comfort of the house for weeks, even months, so you can’t just dump them into their beds outside and call it a day.
Gradually increase the time they spend outside; put them in direct sunlight for short periods of time, then bring them back in. Give them breaks from the wind. Be patient with this process, and if possible, do it on a week of cloudy days or at least a couple days before you plant outdoors.
It’s tempting to leave plants out overnight; I wouldn’t recommend it though. If you live in an area that gets hard freezes at night, absolutely bring your seedlings inside overnight until you are sure they are ready for life outdoors permanently! And be wary of strong winds when hardening off as well; even if there’s no chance of frost in your forecast, wind can cause serious damage to tender seedlings (and mature plants too!).