ORGANIC GREENHOUSE TOMATO PRODUCTION: A PRIMER

In order to succeed as an organic grower, of greenhouse tomatoes or anything else, you first have to be a good grower. In other words, many of the necessary skills and techniques are neither organic nor conventional. These include: building the greenhouse (structure, heating, cooling, irrigation); managing the environment (light, temperature, soil moisture, CO₂); plant management (variety selection, transplant production, grafting, pruning, pollination); and marketing.

In a few of these areas some small changes may come with organic production, such as:

• the greenhouse structure cannot include pressure-treated wood where it can come in contact with crop roots;
• there are relatively few organic fertilizers that can be dissolved and applied through a drip irrigation system;
• use of organic seeds is required, unless you can document that the variety you want is not “commercially available”; and
• marketing organic products requires labeling that conforms to federal guidelines.

The areas where organic production differs significantly from conventional are: type of growing media; sources of plant nutrients; and pest management. The difference has to do with what kinds of media, fertilizers, and pesticides are allowed on organic farms. 

The national organic standards (www.ams.usda.gov/nop) do not allow the use of any ‘prohibited materials,’ which includes most synthetic fertilizers and pesticides. Exceptions to this prohibition are on a national list of ‘approved synthetics’ that includes materials like insecticidal soap and sulfur. There are also some products on the national list of ‘prohibited natural’ materials, such as arsenic and nicotine.  

All the materials in any product to be used on an organic farm must be allowable under the national standards, including so-called ‘inert’ ingredients. As a result, some allowable materials, such as rotenone and B.t. for potato beetle control, are not currently available in an approved organic formulation. The OMRI (Organic Materials Review Institute) seal on product container signifies it is OK to use in organic production (see: www.omri.org).  However, many brands of fertilizers, pesticides and other products may be allowable but have not been reviewed by OMRI. For these, always check with your certifying agency to get approval before using them. Use of a prohibited material may result in decertification. 

Organic greenhouse tomatoes are usually produced using ‘solid’ media rather than hydroponic culture. The system of culture is often in-ground, but can be in beds, buckets, or bags. The rooting media may contain topsoil, or it may be soil-less. Compost, peat, perlite, sand and vermiculite are the other common rooting media ingredients.Compost also acts as a slow-release source of nutrients. Compost in the rooting media must be made according to an approved organic process, or else a delayed harvest requirement of 90-120 days after planting may apply. 

Some combination of organic fertilizers is usually added to the rooting medium to assure the necessary supply of plant nutrients. Testing the nutrient status of the rooting medium is best done with the saturated media (potting mix) test rather than a field soil test, since the rooting media usually very high organic matter.  

Commonly used organic fertilizers include:

• calcitic or dolomitic limestone (Ca, Mg)

• greensand, potassium sulfate (K) or sul-po-mag (K, Mg)
• rock phosphate or bone meal (P)
• blood meal, Chilean nitrate (N)
• plant meals such as alfalfa, peanut and/or soy (N, P, K)

Trace elements are usually provided in sufficiency by compost and/or plant meals, but that can be supplemented using natural materials (volcanic minerals) and synthetic compounds (chelates, sol-u-bor, etc.) that are allowed under organic production, but in some cases only if a deficiency has been demonstrated by soil testing. 

Organic (and conventional) disease, insect and weed management starts with prevention through exclusion, sanitation, and cultural practices that interrupt pest life cycles. If prevention fails, then biological pest controls and organic pesticides may be required. 

Biological pest controls (predators, parasitoids, etc.) are important in organic production to avoid pesticide use. To be effective, biological controls must be introduced when pest populations are still low. Rigorous scouting is key to identifying pest problems early.  

Commercially available biological controls for insects include:

• Aphidoletes, Aphidius, ladybugs (aphids)
• Amblyseius (thrips)
• Encarsia (whiteflies)
• Phytosieulus (spider mites)

Organic pesticides must be used according to the label; common materials include:

• Azadarachtin = neem (many insects)
• Beauvaria bassiana (many soft-bodied insects)
• B.t. israeliensis (fungus gnat larvae)
• Copper hydroxide (many foliar diseases)
• Insecticidal soap (soft bodied insects)
• Pyrethrins (many insects)
• Spinosad (caterpillars, leafminers,thrips)
• Sulfur (two-spotted spider mite, russet mite, powdery mildew)
• Trichoderma (root and foliar disease suppression)

Presented at the Greenhouse Tomato Conference, sponsored by University of Connecticut Cooperative Extension System, NE SARE, and UMass Extension on Nov. 10, 2005.

Vern Grubinger
Vegetable and Berry Specialist
University of Vermont Extension
Brattleboro VT 05301-3669
vernon.grubinger@uvm.edu

Information on our site was developed for conditions in the Northeast. Use in other geographical areas may be inappropriate.

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