Propagation/Greenhouse Management
Part 1 – 106 | Unit 1.3
Lecture 2: Managing Environmental Conditions
Note: Optimal temperatures for germination and subsequent growth differ for some
crops (e.g., Brassicas) and thus germination might be optimized in a greenhouse,
while seedling growth might be best in a hoophouse or outdoors, depending on your
growing environment (contrast Appendix 3 with Appendix 5, Approximate Monthly
Temperatures for Best Growth and Quality of Vegetable Crops)
b) Maintaining good air circulation continues to be important in order to manage
temperatures, prevent diseases, and promote strong structural/cellular development
c) As seedlings develop, irrigation frequency typically decreases, but the depth/volume of
water delivered at each irrigation increases to support the expanding root system and
leaf canopy, and the increased transpiration rate. Reduced frequency and the resultant
wet/dry swing help prevent damping off damage and promote the beginnings of
drought tolerance as crops adapt to cope with short-term moisture limitations.
d) Sunlight is critical for developing seedlings to manufacture nutrients through
photosynthesis and to promote strong cellular growth and compact architecture.
While germination can take place in the absence of sunlight, for example in a growth
chamber (see below), healthy seedling development depends on adequate sunlight;
otherwise, crops will be weak and “leggy,” and thus less able to withstand the more
variable conditions encountered in the ground. In the hottest climates, full sun exposure
can cause tip burning and seedlings may require some shading, but eventually they will
need full exposure to prepare for in-ground life.
- Managing seedling maturation and hardening off: Mature seedlings will typically have
a balance of root and shoot growth—at least two sets of true leaves and an ample root
system that holds together the root ball when removed from the growing container.
“Hardening off ” is the final step in preparing seedlings for transplant and uninterrupted
growth. In the final 3–10 days in the greenhouse zone, seedlings should be outdoors
and exposed to conditions that most closely resemble their future home in the ground.
This acclimatization process reduces transplant shock, which can occur when seedlings
experience an abrupt transition from the protected environment of the greenhouse to the
less predictable conditions of a garden or field setting. During the hardening off process,
the following developments occur, which better enable plants to transition seamlessly to
their new homes in the ground:
a) Full exposure to natural day-night temperature fluctuation promotes a buildup of
carbohydrate reserves. When transplanted, reserves provide seedlings with a nutrient
buffer while they develop new roots to tap into soil resources.
b) Full exposure to stronger air circulation and prevailing wind patterns promotes cell
walls thickening, improving transplants’ ability to withstand the vagaries of the outdoor
environment
c) As plants approach seedling maturity, water is typically delivered less frequently, but in
greater volume
i. Reduced irrigation frequency supports the hardening off process and plants’
transition into the ground. Once in the field, seedlings normally must be able to
withstand longer periods between irrigations.
ii. Consistently providing water to the depth of the containers facilitates root
development and nutrient access across the full volume of soil available to seedlings,
thus maximizing development potential
d) Exposure to full sunlight, equivalent to future in-ground exposure, aids maturing
seedlings in cell development, cell wall strengthening and enhanced photosynthesis.
As with the benefits of the above-mentioned treatments, seedling exposure to full sun
conditions is another aid in reducing the potential for transplant shock.