Before planting your precious seedlings into the garden, they need to be “hardened off”. This term literally refers to the toughness of the stem and entails a short period of acclimatization. Hardening off prepares the plants for fluctuation of temperatures, strong sunlight, uneven moisture and the wind. Hardening off your annuals ensures strong and productive plants to start your garden.
If you have raised your seedlings yourself, there are a couple of hardening off process hints you may begin while plants are still on your windowsill, under lights or in your greenhouse. One is brushing your hand lightly over the tops of the seedlings to simulate a calm wind. This will strengthen the stems. You may also hold back a little on the frequency of watering to allow the roots to go deeper, but not so much to allow wilting of the plants. It is at this point that I pot mine up into 3” pots and so this procedure seems to be automatic. A third suggestion is only if you have started your seedlings en masse in trays. This is called blocking. Using a sharp knife, cut between the plants. This procedure severs lateral roots and forces new branch roots. This minor setback ultimately strengthens the plants and is best done about 10 days before planting out.
The next and most important procedure is the transition of plants from their controlled environment to the variable outdoor climate. As this process completely depends upon weather, the exact suggestions stated here are general guidelines only. Approximately 7-10 days before planting out, or mid-May for Northumberland, choose a calm, warm day and place your seedlings in a sheltered shady spot outdoors for about an hour and bring them back in. The next day try them for two hours. On the third day, try a little full sun for one hour and shade for two. Do not let them dry out and wilt. Lengthen this every day and by the end of the week if there is no frost in sight, you may leave them out all night or transplant them into your garden.
A second method is to use a cold frame. You may buy or make an official wooden cold frame with a hinged clear lid or you can improvise and make your own temporary one. You can use a bottomless wooden box, soil berms, or straw for sides and an old window or plexiglass sheet for the lid. I’ve even used our outdoor glass table and made sides out of scap sheets of wood. Position the structure so that the glazing slopes gently to the south. Inside the temperatures should keep between 10-20°C and checked at least twice a day, opening the lid partially or fully when warm. By the end of the week, the cold frame should be able to remain open at night as well. Sunlight may be monitored by shading the structure. If a frosty night were to occur, the frame could be covered with straw, old carpets or blankets.
Be careful not to “over harden” your plants by hurrying them to full sunlight where their leaves may turn white. Also be careful with temperatures they cannot manage, usually less than 7°C is too much for seedlings. These mishaps would only slow their growth and/or produce lower yields later in the season.
authored by Suzi Gabany