Harden Off Seedlings
Fragile little seedlings all cozy under a warm grow light need to acclimate slowly to the outdoor environment
Hardening off your seedlings is not complicated, it just takes patience. Young seedlings have a thin cuticle that is not ready to protect the leaves and shoots from harsh outdoor conditions when sprouted indoors. The cuticle needs to thicken so that the plant can protect itself and retain water once planted in the garden. We need to toughen them up slowly so the shock of harsh light, wind and sun does not seriously weaken them.
Slowly exposing them in small time periods to wind, sun, rain and even cool nights will allow your seedlings to thicken up that cuticle a little every day. Plan on taking 7 to 14 days at least to complete the process. Sometimes the weather may be too extreme for fragile seedlings to go outdoors, delaying the process a bit. But it is much better to take a few extra days than to lose plants that you have nurtured from seed.
Hopefully your seedlings have developed 3 or 4 sets of true leaves a couple of weeks before the last frost is expected. That is typically a good strong size to start hardening off and transplant.
Select a shady spot protected from sun, wind, squirrels, birds, any other critters that may disturb your seedlings.
On a warm day at least 45 degrees, set the seedlings out in a protected area for one hour. Do not expose them to direct sun or wind yet. Bring them back in to a warm space after the hour.
Each warm day you may increase the time by another hour. If the temperature is not warm enough it is important to just skip the day.
After a few days you can start exposing the seedlings to filtered sunshine and a bit of wind. This exposure should be very gradual. Remember that direct sunlight too soon will scorch the leaves until the cuticle is good and tough.
As you leave the seedlings out longer and more exposed to sun and wind, remember those little seed pots are small and may dry out quickly. Be sure to keep the soil moist.
Seedlings will also need to slowly acclimate to cool nights before you transplant them. Toward the end of your hardening off process, if overnight temperatures remain over 50 degrees, you can also leave your seedlings out overnight. Cool season plants will not mind cool nights but warm season plants like tomatoes and peppers can be damaged if overnight temps drop too low.
After your seedlings have gradually reached full exposure and overnight temps are consistently warm, you can transplant them into the garden or pots. They will go through some shock so try to transplant them on a cloudy morning.
Be patient so you have strong healthy plants to transplant.