Hardening off is not hard to do. About 1-2 weeks before you wish to plant, move the seedlings outside to a shady area, well protected from cold, sun, wind, and heavy rains. Also, protect them from bunnies. Start with an hour or two; then each day, expose the plants to more and more full-sunlight. Watch that the plants do not get sunburned. Restrict the watering a bit, but don’t let the plants wilt. Reduce fertilization, until the plants are in the garden. If the temperature threatens to dip below 45 degrees, bring the plants indoors. Bring the plants indoors at night for the first week, then let them spend the nights outside if it is warm enough. Remember, hardening off is a gradual process; too extreme a transition to outside conditions can shock your plants, set them back a few weeks in their growth, and even kill them.
Hardening off does reduce the growth rate. Carbohydrates and nutrients increase in the plant storage tissues. The cell walls strengthen. The soft and succulent growth becomes firm and tough tissue. The cuticle, or waxy layer on the leaves and stems, thickens and becomes more resistant to evaporation. Root development is enhanced.
Hardening off can also be done in a cold frame or a low hoop house. My wife, Jennifer, bends UV-resistant electrical conduit into semi-circles and sets the ends into the garden soil. Then she connects the tops of the hoops together with cording to hold the structure more rigid. The entire hoop house is covered with plastic sheeting. As temperatures get milder, she daily raises the plastic sheets covering the sides to let out excess heat and allow breezes in. The plastic is lowered each night to protect the plants from cold temperatures and hungry bunnies. If really cold weather threatens, tarps and blankets are placed over the top for added protection. Milk jugs or buckets filled with water collect solar energy during the day and release it at night.
When the plants are able to spend the full day in direct sunlight, and have slept outside for a while, they are ready to be planted out into the garden. If possible, dig and enrich the soil a week ahead of time, and let it settle. Look ahead at the weather to make sure no frost is threatening in the next three nights. To reduce transplant shock, moisten your seedlings about an hour before planting out. It is a good idea to plant seedlings out in your garden on a cloudy day or in the evening, so that they do not get full exposure to the sun until the next day. Dig your hole twice the size of the plant container. Add a bit of compost or slow release fertilizer to the hole. One Master Gardener recommends adding water to the hole, letting it drain, and then planting into it. Tap the plant out of its container. If the roots are encircling or crowded together, gently tease them apart and spread them out into the planting hole. Move soil back into the hole, supporting the new plant; press gently to firm the soil around the plant. It is better not to grab a seedling by the stem as permanent damage may occur to the stem or growing tip; the young leaves are much more durable and expendable. Pay attention to the final plant size, and space the plants at the recommended distance for that variety. Plant tomatoes more deeply than they were in the pot—roots will form along the buried stem. This is not true of other vegetables and most flowers except for marigolds.
Create a small ring of soil around the new transplants to create a shallow well to catch water. For future reference, label the plants at this time. Add plant supports if needed. Add a fence to keep out rabbits or deer if necessary, and a paper collar wrapped around the plant to discourage cutworms. Use an organic mulch to reduce future watering needs and to control weeds. Temporary shade, provided by a cap, a bottomless plastic vinegar jug, or other structure, might reduce wilting at the time of transplant. Water the newly planted seedlings immediately with a weak, soluble fertilizer solution and baby the plants until the roots grow into the garden soil. Floating row covers, “walls of water”, bottom-less milk or vinegar jugs, berry baskets, coffee cans, or similar items can protect the new plants for the first few days from wind, sun, or frost.
Be careful when planting out seedlings grown in biodegradable pots of composted cow manure, peat, newspaper, or other organic material. Break up the bottoms of such containers and score the sides, as they often do not decompose completely and then constrict the roots. The plant roots need to grow out into the surrounding soil. If there is a plastic mesh surrounding the peat container, remove it. Bury the edges of these type of containers below the soil surface when planting, as otherwise they tend to wick and evaporate water out of the soil and away from the plant. The edges could be trimmed shorter if necessary.
One last item: Take the time to enjoy and smell the flowers, and to cook up some fantastic vegetable-herb meals.
Happy Gardening, Joe Baltrukonis