MSU Extension Floriculture & Greenhouse Crop Production

Late autumn is typically a quiet time for greenhouses in Michigan. While many operations aren't expecting plant material for another six to eight weeks, some propagation rooms are already showing signs of life. With winter upon us, now is a great time to think about how we're going to control pests in our spring crops.

We often hear that good cultural practices are the foundation of every successful pest management program, but what does that actually look like? Here we consider several different ways to help us start with cleaner greenhouse crops and reduce the risk of new infestations throughout the season.

Death, taxes… and weeds

Few things are more dependable than weeds. We can predict with almost certainty that they'll show up in our fields and our greenhouses sooner or later. It's certainly not the most glamorous job and generally not one that most people look forward to doing, but consistent weed removal is an easy way to prevent them from becoming future sources of pests and diseases. Consider building in some time to walk down every path in the greenhouse once every two to three weeks. Carry trash bags and make sure all of the weeds are thrown into the dumpster outside the greenhouse. Leaving a pile of weeds in an uncovered garbage can inside the greenhouse gives thrips and other pests an opportunity to escape back into the crop.

Weed barriers

Speaking of weeds, are any of your weed barriers getting old? Did you have a disease problem last year? Growers can easily recognize when a weed mat is past its prime, but it's much harder to tell if they're harboring infectious diseases. Many pathogens have life stages that help them persist in harsh environments, so replacing the weed barriers in areas that experienced heavy disease pressure or were infested with a particularly challenging pathogen (e.g., tobacco mosaic virus) is a relatively inexpensive way to reduce the risk of another disease outbreak.

Cleaning and sanitizing

After removing weeds and replacing old or otherwise undesirable mats, it's a good idea to deep clean and sanitize the greenhouse. Is there a difference? You bet! Cleaning is when you remove the "big" stuff—dirt, algae, dust, plant residues, etc. When you follow up with a sanitizer, you're going after the "small" stuff: pathogenic microbes. The primary benefit from all this is that we significantly reduce the risk of season-to-season transference of insect pests and diseases.

When we remove all the big stuff, what we're really doing is removing food, refuge and breeding sites for insects and pathogens. It's all about reducing the risk of re-infestation. All that big stuff can carry leftover diseases, algae and decaying organic matter attract fungus gnats and shore flies, and both of those pests can transport certain diseases throughout the greenhouse.

A couple of extra thoughts on cleansers and sanitizers:

• Consider using a combination of tools to deep clean your growing spaces. Shop vacuums and brooms can help you clean up the sidewalks and brush off walls, pipes, heaters and other structural objects. Power washing with cleansers or soaps comes highly recommended, but don't forget to rinse off the surfaces when you're done. Many of those products are caustic and can eat away at internal structures.
• Choosing from the plethora of commercially-available greenhouse sanitizers can feel like a daunting task. Those of us that have a difficult time wrapping our head around all of the options may appreciate this guide to Sanitation for Disease and Pest Management from Purdue Extension. The authors provide a detailed discussion on greenhouse sanitation while explaining the differences between various products.

Soil treatment

Many thrips species pupate in the soil. When plants are small, thrips pupae are most likely to be found in the growing media. As plants start to grow wider than their containers, more pupae will find their way to the ground. It's easy to imagine that pupae from hanging baskets, weeds and containers at the edge of the mat could eventually find their way around the weed barrier and into the dirt. At this point the earth inside the greenhouse becomes a source of thrips.

For hoop houses or ranges with consistently high thrips pressure, a soil treatment may be an option to consider. Treatment recommendations for these situations in previous years have included the use of products containing chlorpyrifos or imidacloprid at grub rates. For operations with hundreds of thousands of square feet under protection, the economic and practical benefits of large-scale soil treatments are not completely clear.

Quarantine area

Cutting suppliers and rooting stations spend a lot of time and money on their insect and disease management programs. Despite their best efforts, it's almost impossible to catch every single microscopic invader. When plugs and liners roll off the delivery truck and into our receiving bay, where do we put them? If we're not careful, those carts could end up sitting near production areas where any hitchhiking pest has access to our clean crops. Some growers have found it helpful to set aside an area where new plant material can be isolated and observed for a short period of time. The simple act of placing some yellow sticky traps in the rack and checking them in the morning can help you reduce the risk of introducing pests into the production area.

Prevention and sanitation are the key philosophical concepts that help reduce the risk of insect and disease outbreaks in the greenhouse. By starting with the final crop in mind and staying as clean as possible, we may be saving ourselves quite a few headaches at shipping time.