Dormant Greenhouse - A Good Time to Clean Up

Stanton Gill

Winter is here and for most cut flower growers it the time of year to plan the 2012 season. Those of you who shut down your greenhouse or hoophouse in December need to clean up and prepare this winter to prevent insect and disease problems before you start spring crops.

If you are overwintering cool-season plants such as sweet pea, snapdragon, or pansies in minimally heated houses, you will need to monitor them in March and April. Female aphids may be settled in on these crops and will start cranking out nymphs when temperatures warm. Make sure you control them early in the season before they spread onto your new spring plants.

If you heat your green-houses, two-spotted spider mite populations can blossom in the low humidity, especially on plants growing on the south and southwest sides of the structure. This is most common February through April.

If you’re starting the spring with empty greenhouses or hoophouses, you have the best chance to get a jump on insect and diseases by walking through a general cleanup process. Otherwise, you’ll be too busy when spring arrives to get back to this task.

Can You Start Clean and Stay Clean?

After each cropping cycle take the time to completely clean the greenhouse of all plant material and growing medium, debris, and weeds. This removes harborage for insect and mite pests, and diseases. Use a power washer on benches and floor surfaces. After all plants have been removed from the greenhouse use a leaf blower to either blow out or vacuum leaf debris. Although a rapid cleaning method, this may result in dust covering the inside surface of the greenhouse glazing, which will have to be removed in order to avoid a reduction in light transmission.

Remove all weeds from inside and outside the greenhouse, particularly near sidewalls, doorways, and intake vents. Fabric weed barrier makes an efficient reduction method. Although weeds may germinate through the weed fabric, they can be easily pulled and discarded. In an organic greenhouse you can plan on hand weeding and physical removal of weeds as part of your routine. If you have concrete floors weed control is a breeze.

Avoid leaving residual plant material from the previous cropping cycle in the greenhouse. Keep in mind that the longer plants remain in the greenhouse the greater the probability of having to contend with pests. Residue from old plants can serve as a reservoir for insects, mites and diseases that may spread to the new material.

Clean or disinfect all walkways and greenhouse benches with Green Shield or dilute solutions of Clorox (Clorox brand is the only sodium hypochlorite with an EPA disinfectant label). Hydrogen dioxide and hydrogen peroxide generally are approved for use in greenhouses.

Eliminate all algae from the greenhouse. Algae can build up on floors and benches, and serves as a food source for fungus gnats and shoreflies. Quickly deal with low spots or poorly-drained areas in the greenhouse that allow water to accumulate. Avoid over-fertilizing plants with nitrogen-based fertilizers.

Routinely check horizontal airflow fans (HAF) to make sure they are functioning properly. These work by distributing the air mass throughout the greenhouse, useful in maintaining even temperatures within the space,  improving air circulation, and reducing the incidence of Botrytis.

Winter is a good time to calibrate soluble salt and pH meters. Calibrate fertilizer injectors at least once during the growing season, and test irrigation water for pH and alkalinity. With organic fertilizers you will need to establish the correct setting on your injector but you can measure soluble salt levels and pH just as you would with chemical fertilizers.

Order yellow sticky cards. If possible, establish thresholds for insect and mite pests so that appropriate pest management strategies may be implemented. There is no general threshold number for pests above which they may occur. Experience will be helpful in establishing thresholds for specific crops. Thresholds will vary depending on the insect pest and whether this pest vectors diseases.

Take stock of what pesticides you have on hand and decide which materials will be used if you detect a pest during the season.
The final step is to hope for a warm spring and pray for nice weather from April through Mother’s Day to help weekend sales.

Pesticide Mixtures

Raymond Cloyd

A pesticide mixture is the combi-nation of two or more pesticides, such as an insecticide and/or miticide, into a single spray solution. These combinations expose individuals in an arthropod (insect and/or mite) pest population to each pesticide simultaneously. Pesticide mixtures may be more effective against certain life stages including eggs, larvae, nymphs, and adults of arthropod pests than individual applications, although this may vary depending on the rates used and formulation of the chemicals used.

There is already widespread use of this practice, partly because combinations of selective pesticides may be required in order to deal with the arthropod pest population complex present in the crop. Typically, two pesticides are mixed; however, it has been demonstrated that three or more pesticides may be combined into a spray solution to target different insect and/or mite pests. This article discusses the benefits and potential problems associated with these combinations and how they may alleviate resistance.

Benefits of Pesticide Mixtures

Pesticide mixtures may enhance arthropod pest population suppression as a result of synergistic interaction or potentiation between or among combined chemicals. Synergism refers to the toxicity of a given pesticide being enhanced by the addition of a less or non-toxic pesticide, or other compound such as a synergist (e.g., piperonyl butoxide), which is used to increase the effectiveness of an insecticide application. Potentiation involves an increased toxic effect on arthropods, when two compounds are mixed, which by themselves are harmful to these pests.

The main benefit of mixing pesticides is a reduction in the number of applications required, which decreases labor costs. Further, mixes may result in higher mortality than if either pesticide were applied separately. Studies have demonstrated that these mixes increase efficacy against pests such as whiteflies, and western flower thrips (Frankliniella occidentalis) compared to separate applications of each pesticide.

Many studies have evaluated the effects of pesticide mixtures in suppressing populations of agricultural pests, whereas there is minimal information associated with horticultural cropping systems including cut flowers. However, it has been demonstrated that all two-, three-, and four-way combinations of abamectin (Avid), bifenazate (Floramite), azadirachtin (Azatin), and imidacloprid (Marathon) along with spinosad (Conserve) did not affect suppression (based on percent mortality) of western flower thrips.

This indicates that antagonism is not an issue with any of these mixtures. It was also found that nearly all the two- and three-way combinations associated with acetamiprid (TriStar), bifenazate (Floramite), buprofezin (Talus), and chlorfenapyr (Pylon) exhibited no antagonistic activity with nearly all the blends efficacious (based on percent mortality) against populations of the sweet potato whitefly B-biotype (Bemisia tabaci) and the two-spotted spider mite (Tetranychus urticae).

Potential Problems with Pesticide Mixtures

Along with benefits, potential problems must be considered. These include plant injury (phytotoxicity), pesticide incompatibility, and antagonism. Antagonism occurs when the mixture of two or more pesticides results in reduced efficacy (based on percent mortality) compared to separate applications of each, or when the combined toxicity of two materials applied together is less than the sum of the toxicities of the materials applied separately. As such, antagonism may compromise the efficacy of insecticides and/or miticides. 

Incompatibility is a physical condi-tion in which pesticides do not mix properly to form a homogenous solution or suspension. Instead, flakes, crystals, or oily clumps form, or there is noticeable separation. Incompatibility may be affiliated with chemical and/or physical properties of the pesticides, impurities in the water, or the types of formulations being combined. To determine incompatibility (or compatibility) of a mixture, a ‘jar test’ should be conducted in which a representative sample of a pesticide mixture solution is collected in a glass jar and allowed to remain stationary for approximately 15 minutes. If the solution is uniform or homogenous, the pesticides are compatible; however, if there is clumping or separation, the pesticides are not compatible.

Pesticide Mixtures and Resistance

It has been proposed that pesticide mixtures may delay the onset of resistance developing in arthropod pest populations. Implementing pesticide resistance management strategies is important in preserving the effectiveness of available pesticides. However, minimal evidence suggests that pesticide mixtures may actually alleviate the onset of resistance.

Mixing pesticides with different modes of action may delay resistance developing within pest populations because the mechanism(s) required to resist each pesticide in the mixture may not be widespread or exist in those populations. As such, it may be difficult for individual pests to develop resistance to several modes of action simultaneously. Those arthropods resistant to one or more pesticides would likely succumb to the other chemical in the mixture as long as pesticides with different modes of action are mixed.

As such, it is important to combine only pesticides with different modes of action or those that affect different biochemical processes in order to prevent resistance. For example, acephate (Orthene) and methiocarb (Mesurol) should not be mixed because despite being in different chemical classes (organophosphate and carbamate) both have identical modes of action.

Acephate and methiocarb block the action of acetylcholinesterase, an enzyme that deactivates acetylcholine, which is responsible for activating acetylcholine receptors. This then allows nerve signals to migrate through the central nervous system. Both acephate and methiocarb inhibit the action of acetylcholinesterase by attaching to the enzyme. Similarly, although the active ingredients of the miticides acequinocyl (Shuttle), pyridaben (Sanmite), and fenpyroximate (Akari) are in different chemical classes; napththoquinone, pyridazinone, and phenoxypyrazole, respectively all three are classified as mitochondrial electron transport inhibitors (METI). These active ingredients either inhibit nicotinamide adenine dinucleotide hydride (NADH) dehydrogenase (complex I) associated with electron transport, acting on the NADH CoQ reductase, or bind to the quinone oxidizing (Qo) center or cytochrome bc1 (complex III) of the mitochondria respiratory pathway. This reduces energy production by preventing the formation of adenosine triphosphate or ATP. As such, these designated miticides should never be mixed.


Pesticide mixtures involve combi-nations of two or more pesticides into a single spray solution. These are widely used to deal with the array of arthropod pests encountered in cut flower production systems due to a savings in labor costs. Their use may result in synergism or potentiation (enhanced efficacy) and alleviate the potential for resistance.

However, antagonism (reduction in efficacy) may also occur due to mixing two (or more) pesticides. Judicious use of pesticide mixtures or those that may be integrated with biological control agents or natural enemies is especially important because parasitoids and predators (and even microbials such as beneficial bacteria and fungi) can suppress arthropod pest populations irrespective of the presence of resistant individuals. The use of mixtures to prevent or avoid resistance must not divert attention from implementing alternative pest management strategies including cultural, sanitation, and biological control that can reduce reliance on pesticide mixtures and minimize the potential for pesticide resistance. Pesticide mixtures will continue to be an integral component of pest management programs due to the continual need to deal with a multitude of arthropod pests associated with horticultural cropping systems.

A new book is available from the University of Maryland Extension: Total Crop Management for Greenhouse Production – With an Emphasis on IPM and Nutrient Management. Contact me at [email protected] for a copy. They are $25 each (includes shipping). This 300+ page manual lists all the chemical and biological controls available for controlling insects and diseases in greenhouses.

Raymond Cloyd


Raymond Cloyd is Professor and Extension Specialist in Horticultural Entomology at Kansas State University. For more information, contact him at [email protected]