Introduction
The transplanting process can be a shock to rapidly growing
seedlings especially when set out into the cold windy garden in the spring.
This is especially true for transplants started in the greenhouse, cold frame,
hotbed or home. These young seedlings can be made somewhat resistant to heat,
cold temperatures, drying and whipping winds, certain types of insect injury,
injury from blowing sand and soil particles and low soil moisture by a process
termed “hardening.”
The term “hardening” refers to any treatment that results in
a firming or hardening of plant tissue. Such a treatment reduces the growth
rate, thickens the cuticle and waxy layers, reduces the percentage of freezable
water in the plant and often results in a pink colour in stems, leaf veins and
petioles. Such plants often have smaller and darker green leaves than non-hardened
plants. Hardening results in an increased level of carbohydrates in the plant
permitting a more rapid root development than occurs in non-hardened plants.
Cool-season flower and vegetable plants can develop hardiness
allowing them to withstand subfreezing temperatures. Unhardened cabbage
seedlings have been reported to be damaged by temperatures of -2 degrees C (28
degrees F) while hardened cabbage will tolerate temperatures as low as -6
degrees C (22 degrees F).
Warm-season types of plants even when hardened, will not withstand
temperatures much below freezing. If transplanted to the garden or field prior
to the average last killing spring frost, such plants should be provided
protection by hot caps or other such devices.
Method
Any of the following can be used to harden transplants. A
combination of all these techniques at one time is more effective.
·
Gradually reduce water – water lightly at less frequent
intervals but do not allow the plants to wilt severely.
·
Expose plants to lower temperature than is reported as
optimal for their growth. If biennials are exposed to cold for an extended
period, they may bolt in lieu of developing properly.
Note: Placing the
plants outside during the day to encourage hardening and then bringing the
plants back into the warm house during the night often reverses the hardening
process. Plants could be placed in a cold frame or other area that does not
freeze during the night hours without loss of the hardening process.
·
Do not fertilize, particularly with nitrogen immediately
before or during the hardening process. A starter solution or liquid fertilizer
could however be applied to the hardened transplants one or two days prior to
transplanting into the garden or at the time of transplanting.
·
Gradually expose the plants to more sunlight. This
results in the development of a thicker cuticle layer thereby reducing water
loss.
Cautions
Hardening is not necessary for all transplants. We recommend that
with the exception of tomatoes, plants that are susceptible to frost should not
be hardened. Overly hardened plants while withstanding unfavourable outside
conditions are slow to get started and may never overcome the stress placed on
the plant during the hardening process. We also recommend that plants be
hardened for no longer than seven to ten days before planting to the garden
site.
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