Propagation/Greenhouse Management
Part 1 – 100 | Unit 1.3
Lecture 1: Seed Biology, Germination, & Development
c) Perennials: Plants that live more than two years. Once beyond their juvenile life phase,
perennials grow vegetatively, flower, and produce seeds every year. The life span of
perennials depends on the genetics of the species and the environmental conditions
in which the plants are growing. By definition, perennials can live three to thousands
of years, but lifespan within a particular species tends to vary. Perennials can be grown
from seed, although many are reproduced asexually/vegetatively to hasten maturity,
maintain genetic uniformity, and therefore retain desired morphological characteristics.
- Open pollinated (OP) and hybrid seed (see also Appendix 1, Characteristics of Open-
Pollinated (OP) and Hybrid Seed)
a) Open-pollinated seed: Produced when a parent plant is fertilized by another member
of the same genetically stable population. Offspring bear traits or qualities that closely
resemble the parent population. These seeds may come from:
b) F1 Hybrid seeds: The product of cross pollination of two different, but homogeneous
inbred, stable lines, each of which contribute desirable characteristics to the subsequent
generation. Seeds saved from this next generation typically possess a highly
heterogeneous nature and will produce offspring unlike the hybrid parent population.
B. Seed Germination and Early Seedling Development
- Necessary pre-conditions for seed germination
a) Viability: Seeds must contain living, healthy embryonic tissue capable of germination.
i. Viability depends upon the full development of the embryo and endosperm (nutrient
storage tissue) during the development of the seed
ii. Viability is also contingent upon maintaining the health of the embryo and
endosperm from seed maturation through seed sowing. Moisture within the seed,
nutrient reserves, and an embryo’s potential to germinate are finite, as determined by
the genetics of the species and by the environmental conditions during seed storage.
See Appendix 2, Seed Viability Chart, for typical lifespan of common vegetable seeds.
b) Many species also exhibit dormancy factors that inhibit or delay seed germination.
Dormant seed cannot germinate under what would otherwise be conditions favorable
for germination until dormancy factors have been overcome. Physical and chemical
dormancy are more common in native species and plants from more extreme
environments than in commonly grown vegetable and flower crops.
i. Physical dormancy (e.g., hard, thick seed coats): Can be broken by soaking, scarifying,
exposure to soil microorganisms. Methods are species specific. (See Resources
section for guides to propagation techniques.)
ii. Chemical dormancy: Growers replicate natural processes and environmental
conditions to break internal chemical/metabolic conditions preventing seed
germination (e.g., leaching, cold/moist stratification, fire scarification, etc.)
- Environmental factors involved in germination are typically both atmospheric and edaphic
(soil related). Biotic factors, such as pests, pathogens, weeds, and microbes can also be
involved.
a) Temperature: For ungerminated seed, temperature is normally discussed in reference
to soil temperature. All seeds have minimum, maximum, and optimal soil temperature
ranges within which germination is possible (see Appendix 3, Soil Temperature
Conditions for Vegetable Seed Germination).
i. Minimum: Lowest temperature at which seeds can effectively germinate. As
compared to temperatures in the optimal range for a given species, days to
emergence will be long, percent germination will be low and rate of subsequent
growth will be slow when temperatures approach the minimum threshold for a given
species.