Hardening off seedlings is the process of readying young, greenhouse nursery plants for planting in an outdoor bed. Hardening off is very easy to do and usually takes no longer than a week.
So why harden off seedlings in the first place? Greenhouse plants are raised in an optimal environment with controlled humidity and temperature levels, and protection from the blazing sun and night time frosts. Moving seedlings directly from this controlled environment to the outdoors can shock the plants, especially if transplanting in warmer, more arid climates. The shock of the outdoors can scorch tender leaves or dry out fragile root systems, both which can kill those expensive seedlings that have been purchased for the garden.
Hardening off provides the seedlings with a period of adjustment as they get used to the outdoors.
How to harden off seedlings
Hardening off seedlings usually starts about a week before the last spring frost. The basic ideal behind hardening off is to introduce the plants to sunlight incrementally. For the first day, the plants can be placed in a protected area with filtered sunlight, such as beneath a shade tree. During the course of the week, the seedlings are then set out in the morning sun for short periods of time and gradually worked up to full days of sunlight. By the end of the week, the seedlings can be planted into the garden.
A couple of points worth noting here. Regular, deep morning watering is a must during the hardening off period since potted plants dry out quickly. And, since frost danger is still possible, the seedlings must be brought indoors at night during the week.
Protecting tender seedlings from harsh, desert heat or arid climates.
If you live in the desert like I do, hardening off seedlings isn’t enough to prevent the transplants from going into shock. Transplanted seedlings need an extra measure of protection from the heat of the sun and reduced humidity levels. Here’s how it’s managed on our urban homestead:
1. Plant seedlings in the early morning.
2. Once in the ground, water seedlings thoroughly.
3. Pack the area around the seedling with a 2 inch layer of natural mulch such as straw or grass. Remove bits of mulch touching the plant stem. Water again until the mulch is saturated.
4. Water seedlings thoroughly every morning. Deep watering will provide the plants with enough moisture to last through the hottest part of the day.
5. For the first two weeks in the ground, cover the seedlings with an inverted plant pot during the hottest parts of the day. The inverted pot will protect the seedling from the rays of the sun and slow down the evaporation of water. The pots can be removed once temperatures dip below 75 degrees.
By the end of the two weeks, the seedlings will have doubled in size and are ready to tackle the desert heat all on their own.