Propagation/Greenhouse ManagementUnit 1.3 | Part 1 – 105
Lecture 2: Managing Environmental Conditions
Lecture 2: Managing Environmental
Conditions—Using Greenhouses to Optimize
Seedling Production
A. Optimizing Germination, Seedling Development, and Seedling Maturation
The principal role and function of greenhouse facilities is to modify or manage environmental
conditions to optimize plant health and development. Although greenhouse structures
serve many purposes, from producing transplants, to in-ground production of high value
crops, to early and late season extension in a range of climates, this lecture focuses on using
greenhouse facilities for seedling production.
- Optimizing germination: Propagation structures, combined with the knowledge and
experience of the greenhouse grower, can be managed to create optimal environmental
conditions (e.g., temperature, air circulation, light, and soil medium moisture) that facilitate
rapid germination and early crop establishment
a) To promote rapid germination, temperatures must be maintained within the
appropriate range for chosen crops (see Appendix 3, Soil Temperature Conditions
for Vegetable Seed Germination). Temperatures below the optimal range will either
delay germination or promote erratic germination, and thus inconsistent seedling
age. Temperatures above the optimal range can induce thermo-dormancy in some
crops, such as lettuce and spinach, preventing or delaying germination. Temperatures
within the optimal range will promote rapid, uniform germination and consistent early
development.
b) Consistent air circulation is critical for crop health, both to provide adequate oxygen
for respiration and to mitigate against presence of fungal pathogens/“damping off ”
organisms, which thrive with consistent soil moisture and stagnant air conditions
c) With recently sown seed and germinating seedlings, moisture delivery is typically
frequent and shallow. Consistent delivery, combined with high quality soil media,
prevents desiccation of imbibed seeds and emerging root radicals. However, a moderate
wet-to-dry swing in surface soil conditions, especially once crops have germinated, is
critical to prevent the presence and proliferation of damping off organisms.
i. Pythium, Rhizoctonia, Fusarium and Phytophthora, the primary genera of fungal
pathogens known as “damping off ” organisms, can be controlled by managing
environmental and cultural conditions: Allowing for a wet-to-dry swing between
waterings, preventing stagnant air in the greenhouse, promoting consistent airflow,
and when necessary, managing for temperatures that limit pathogen proliferation
- Promoting healthy early seedling development: Ongoing management of environmental
conditions (temperature, air circulation, and moisture delivery) is required as seedlings
develop, but with most species, seedlings’ physiological tolerance expands and precise
environmental control may be less necessary to maintain optimal development. When
greenhouse space is at a premium, young seedlings are typically moved to alternative
structures (from a greenhouse to a hoop house, for example) to make way for the next
generation of crops most dependent on precise environmental control.
a) Temperature management remains critical, especially when trying to extend seasonal
parameters. Growing in the protected/moderated environment of the greenhouse or
hoophouse will promote more rapid development than normally possible outdoors by
creating more favorable daytime conditions and minimizing nighttime chilling of crops
and soils, which will slow the resumption of growth the following day.