Extension Communications
3614 Administrative Services Building
Ames, Iowa 50011-3614
(515) 294-9915
4/5/99
Contacts:
Narjess Zriba, Plant Pathology, (515) 294-0589
Mark Gleason, Plant Pathology Extension, (515) 294-0579
Elaine Edwards, Extension Communication Systems, (515)
294-5168
Yard and Garden Column for the Week Beginning April 9
Get ready to harden off your transplants
By Narjess Zriba
Plant pathologist
Iowa State University
Most gardeners probably have practiced hardening off their transplants without ever realizing that's what it's called. I remember my stupefaction when I was told I needed to harden off healthy-looking tomato seedlings. The use of hardy vegetable transplants is very important for success, especially to novice gardeners. Generally, good quality transplants can be selected by their stocky, healthy appearance, medium size (not too leggy), deep green color and freedom from diseases and insects.
As the name suggests, hardening off is a process by which transplants are toughened up so that they can withstand nature's caprices as they grow. In a way, it's like preparing a child for the real world by "letting go" gradually and carefully without stopping the worry and supervision. It's done for your plant's benefit, and hardened plants will grow better in the future. It gives your plants a better chance to survive the transition phase between the sheltered indoor environment and the garden's less predictable conditions. Vegetable plants can develop hardiness, allowing them to withstand wind, sun, subfreezing temperatures, and fluctuations in moisture. Plants will struggle and may succumb to environmental damages and pests if not hardened off.
Hardening off reduces the growth rate, thickens the cuticle and waxy layers, reduces the amount of freeze-prone water in the plant, and often results in a pink color in stems, leaf veins and petioles. Such plants often have smaller and darker green leaves than nonhardened plants. Hardening results in an increased level of carbohydrates in the plant and triggers more rapid root development than in nonhardened plants.
Transplanting from indoors to outdoors is a profound shock for a plant. Low but non-freezing temperatures affect several physiological processes in chilling-sensitive plants such as tomato, pepper, cucumber, squash and beans. One of the most important alterations concerns plant water status. Due to a poor control of water loss by the leaves and reduced water uptake by the roots, chilled plants show symptoms of dehydration such as withered leaves. Chilling injury can be prevented by hardening treatments such as water deprivation, low temperature conditioning, and mechanically induced stress, or by using chemical protectants. Hardening treatments can be used individually or in combination. Nevertheless, the biological mechanisms involved in the hardening process are not well known.
The conventional method of hardening off recommends that plants be progressively introduced to the outside world by placing them in a somewhat protected and shaded location for a few hours each day over 10 to 14 days before transplanting. This method slows their growth and toughens them up. The transplanting procedure shocks plants, but the shock is a mild and controlled one. Transplants are most likely to recuperate quickly if they are smaller rather than larger when transplanting occurs, so it makes sense to keep them small.
A few days before you start to take the transplants outdoors, water them lightly at less frequent intervals. Tomatoes, for example, can droop but should not be allowed to turn brown and crispy. Do not fertilize, particularly with nitrogen, immediately before or during the hardening process. However, a starter solution or liquid fertilizer can be applied to the hardened transplants one or two days before transplanting into the garden or at the time of transplanting. Next, start by placing them outdoors in a protected location for a few hours per day. During the next 7 to 10 days, gradually increase the amount of time they spend outside and extend their exposure to wind and sun. This forces the development of a thicker cuticle layer, thereby reducing water loss. After several 10- to 12- hour days of exposure to outdoor weather conditions, leave them outside 24 hours a day for a couple of days. Then, they will be ready to transplant to the garden.
You can also avoid hardening off altogether by placing the tender transplants directly in your garden with some form of protection. Some gardeners use juice cans, bottomless plastic milk jugs or commercially available hot caps that offer protection for the transplant during its first week outside. Impatient gardeners or very experienced ones tend to do that. The potential advantage of not hardening off is obviously 7 to 10 days growth gain for the transplants. This potential gain, however, is at the risk of the plant's life itself. Some plants can be easy victims of the harsh environment in the garden, especially tender crops such as basil, tomatoes, peppers and cucurbits.
Whether you choose to harden off or avoid it, the next step is what gardeners enjoy the most: watching the young plants grow and bear something good to eat.
ml: isugarden
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