Horticulture Principles and Practices

Temperature
The soil should be kept warm for good germination by most seeds. Place the flat in a
warm place at 20 to 26°C (68 to 79°F), depending on the species. Even though 21°C
(70°F) may be adequate, a warmer temperature of about 24°C (75°F) will cause germination
to occur more rapidly. Certain seeds such as pansy and snapdragon prefer a relatively
cooler germination temperature of about 18°C (65°F).
Temperature fluctuations in the seedbed are caused by evaporation and irrigation
water. Evaporation of moisture may cool the soil surface water by about 1 to 3°C (2 to 5°F).
When cold water 7 to 10°C (45 to 50°F) is used in watering, it can cause the soil temperature
to drop. A cold soil takes a long time to regain its heat. To stabilize soil temperature,
some growers use hotbed cables. Some of the small-scale propagation boxes have heating
coils. In commercial greenhouses, biotherm, or bioenergy, systems or hot water tubes are
used to provide plants growing in containers with heat from the bottom.
Fertilizing
Fertilizers are not required until the seeds germinate. The stored food in the cotyledon is
usually adequate for the period of germination. High fertility in the germination medium
results in weak seedlings. The soil medium should have a good pH (6.0 to 6.8). Phosphorus
is required for strong root development; consequently, the element should be
available as soon as possible to ensure seedling survival.
9.15.6 AFTER-GERMINATION CARE
Fertilizing
Once the seeds germinate, the seedlings should be provided with a low-level application
of a complete fertilizer analysis such as 20-20-20 at the rate of 1 ounce (28.35 grams)
per 3 gallons (11.4 liters) of water. Fertilizer should be applied weekly to avoid the development
of weak seedlings. Weak seedlings are difficult to transplant and less likely to
survive the operation.
Hardening Off
The process of preparing
seedlings for transplanting
by gradually withholding
water, nutrients, and
decreasing temperature.
Transplanting Shock
A temporary setback in
growth suffered by fresh
transplants due to adverse
conditions.
Hardening Off
Hardening off is a horticultural practice whereby seedlings are prepared for transplanting
to the field. The danger transplants face in the field is the drastic change in the
growing environment from a more controlled one to a harsh one. If unprepared, transplants
are prone to transplanting shock (stress that may be suffered by transplanted
seedlings). This preparative process is called hardening off and is usually accomplished
by manipulating the temperature and moisture levels. Seedlings are gradually
exposed to slightly cooler temperatures (13 to 15°C or 55 to 59°F) and/or reduced
moisture.
9.16 TRANSPLANTING
308 Chapter 9 Sexual Propagation
9.16.1 TRANSPLANTING FROM FLATS
If seedlings cannot be transplanted when they are ready, they should be stored at a
cool temperature to slow their growth (e.g., in a refrigerated environment at about
5.5°C [40°F]). They should be brought into the open for at least a day before transplanting.
Seedlings may be transplanted from the nursery bed directly to the field. If the
grower is raising seedlings for sale, the seedlings are transplanted into containers such
as peat pots or cavity seedling trays and nursed to marketable size. In the second case,
seedlings are transplanted at a younger age and then must be transplanted a second time