Growers must always exercise caution when applying any controlled release fertilizer product to plants that will be under protective cover. Due to plant dormancy and minimal irrigation, a grower must monitor soluble salts and irrigate as needed to avoid plant injury (if irrigation is unavailable, such anapplication is not recommended). Air temperatures even in an unheated poly house can rise above 45° F. This may initiate nutrient release from any CRF left in the pots from earlier applications. Since plants at this point are still dormant, it may be necessary to irrigate crops under protective cover to prevent soluble salt build-up. Many growers want to apply CRF to protected containers in late fall or early winter when labor is readily available and fertilizer can be applied before the onset of the busy spring shipping season. If CRF is to be applied under winter protective cover, the grower must be in a position to irrigate and monitor soluble salts regularly
To prevent problems, follow these steps:
- Overwintering under microfoam. Normally, if microfoam is applied over adequately watered plants that have reached dormancy, winter injury will not occur. Release of nutrients from CRF will be slow enough to prevent excessive soluble salts build-up. When plants are uncovered in the spring, a thorough leaching will remove any excess salts that may have accumulated during the winter protection period.
- Overwintering without protection.Nurseries located in areas with milder winters, defined as experiencing no temperatures less than 20° F, often provide no winter cover. Instead, they frequently place plants pot-to-pot and surround beds of more tender species with a row or two of hardy varieties for wind protection. During very cold periods, a nursery may temporarily cover plants with shade cloth or burlap, leaving the cover in place for several days or even weeks to prevent desiccation from wind. These procedures have little effect on nutrient release from CRF. Temperatures usually stay cold enough to keep plants dormant, and it rains frequently enough to keep any accumulated salts moving through the pot.
- Overwintering in plastic houses with irrigation. Many nurseries use plastic Quonset type overwintering structures, normally covering these houses just before the onset of extremely cold weather and uncovering prior to spring warm-up. Most nurserymen recognize the fact that warm temperatures can occur inside these houses during the period when plants are covered, and they water to prevent plant desiccation and the potential for soluble salt build-up from CRF. When applying water be certain to leach pots thoroughly.
- Overwintering under plastic without irrigation. If no provision can be made for watering in these houses during such warm-up periods, consideration must be given to using a fertilizer which will diminish its nutrient release prior to covering in the fall. During the months of winter protection, which usually extend from late fall to early spring, plants will have a reduced nutrient supply, preventing an early growth flush under the plastic. Fall or winter CRF applications should not be made under these conditions. It is not advisable to leave late-potted plants unfertilized for the winter. Lack of nutrients will prevent optimum winter root development, and if plants are not fertilized immediately after uncovering—and they usually are not—lack of fertility will prevent adequate growth during the very important first flush in the spring.