Gardening Under Cover-Plant Production

In Part I of this series, we investigated the greenhouse as a structure or piece of equipment in which to grow plants. This month we take a look at some of the principles involved in greenhouse gardening. While plants basically have the same needs for water, mineral elements and sunlight whether grown outdoors or in a greenhouse, the way by which these needs are supplied in greenhouse does vary from outdoor production.

The vast majority of plants produced in greenhouses today are grown in containers. Container production has many advantages over growing in beds but it does make proper watering more difficult. A primary reason for this is that the growing medium in a container is removed from capillary contact with the soil which makes drainage much more difficult from a container than from a bed. That same fertile garden loam that might produce excellent results in an outdoor (or greenhouse) bed would likely yield disappointing results when used in a container because of what is known as the “perched watertable effect.” In essence, this effect simply makes it more difficult for gravity to overcome the capillary force of a soil when in a shallow pot opposed to a deeper profile of soil in a bed. The result is a medium which remains far too wet too long after watering, thus depriving a plant’s root system of much-needed oxygen and leading to the possibility of root diseases.

The solution to this problem lies in the use of growing media with relatively poor capillarity or water-holding capacity. Soilless growing media containing large particles have much less capillarity than those containing soil which has much smaller particles. While blends of Sphagnum peat, perlite and vermiculite (e.g. 70:15:15 % by volume) would be hard to keep properly watered in a bed because of their inability to hold water against the force of gravity, they are ideal for container production of plants. Additionally, they are considered to be pathogen-free and chemically inert. There are several commercial firms that formulate and package their own brand of soilless media. For the sake of drainage and possible contaminants that might accompany soil, one should be sure that no soil is contained by the mix. This can be determined by inspecting the label rather than the name of the product since several products generically labeled as “potting soil” actually are soilless in nature. Look for Sphagnum peat as one of the ingredients; this type of peat is superior to other types (e.g. Hypnum or reed sedge) often used in cheaper mixes.

The problems solved through the use of soilless growing media unfortunately create new problems relative to plant nutrition. Field soil has considerable exchange capacity which allows it to hold on to some of the mineral elements essential for plant growth. The common practice of applying a maintenance dose of fertilizer prior to planting in a soil bed takes advantage of this fact. Additionally, the breakdown of the inorganic components of soil provides the essential trace elements also needed for proper plant nutrition. Soilless growing media lacks both of these traits, making alternative feeding regimes necessary. Continuous liquid feeding is, perhaps, the most common alternative fertilization method.

Continuous liquid feeding (CLF) involves applying a relatively dilute nutrient solution each and every time the plant is watered. Fertilizers used for this purpose should be instantly and totally water-soluble and also should contain the trace elements necessary for proper plant growth. Those fertilizers that have a significant amount (at least 50%) of the nitrogen they contain in the preferred nitrate form and are labeled as “peat-lite” in formulation are preferred for greenhouse use. The latter can only be determined by checking the label on the fertilizer bag. Feeding at the rate of 200 ppm nitrogen CLF from a fertilizer such as 20-10-20 or 21-5-20 is a starting point for most container-grown greenhouse plants. From there, the rate can be adjusted upward or downward depending upon the preferences of the crop in question. Various syphon devices or injectors are available to make this task easier; if they are used, be sure to install backflow preventers to keep nutrient solution from possibly contaminating the home water supply. Another caution is to never water plants with water-softened water because of the high salt content involved. This can be a problem if hobby greenhouses are attached to home with water softeners.

Pest control involves another major challenge for the greenhouse gardener. Nature supplies the gardener with a system of checks and balances that helps to keep insects and diseases from becoming huge problems in outdoor plantings. We disrupt that system when we set up a very controlled environment in a greenhouse where we often (inadvertently) supply plant pests with everything they need to flourish and exclude their natural predators. Commercial greenhouses employ a system of integrated pest management (IPM) to control their pests; hobby greenhouse owners would be wise to follow such a system also.

The first IPM principle is exclusion of the pest. Good sanitation practices along with insect screening on all greenhouse openings can go a long way toward eliminating the need for pest control measures by excluding pests from entering the greenhouse. Careful inspection of any new plant introduced into the greenhouse is an essential part of exclusion. The second IPM principle is that of surveillance. Plants need to be regularly inspected to try and detect the presence of insects or diseases that might have escaped our attempts to exclude them. Insect monitoring (sticky) cards are available that help make this task a bit easier. Treatment is the third principle of IPM. This implies that pest populations have increased to the point that some sort of curative measure is needed. While biological control is becoming more popular, most greenhouse owners still rely on chemicals to combat pest problems. Newer, safer chemicals are available today that lessen the risk to people, our plants and the environment and should be used as a first line of treatment. One can go on-line or check with the local extension office for a current list of pesticides labeled for greenhouse use. If pest infestation on a particular plant is too great, discarding the plant might be a better management practice than trying to rid it of the pest.

Hobby greenhouse gardening is challenging but rewarding. Recognizing the basic difference between growing outdoors vs. indoors can help meet these challenges and lead to a lifetime of enjoyment working with plants in a very unique setting.

David Trinklein, Associate Professor, Horticulturalist, UMC (573)882-9631