Southwest Michigan greenhouse report — March 2018

Now is a good time to check your water quality and review irrigation practices.

Photo by Jeremy Jubenville, MSU Extension
Photo by Jeremy Jubenville, MSU Extension

Spring crop status

Busy, busy, busy! The advent of spring signals a very hectic time for greenhouse growers in Michigan. While some of the earliest batches of flowering crops are being shipped south, propagation rooms are still very active and production lines are operating in high gear. There are baskets to hang, pots to water and liners to trim. There are schedules to adjust, equipment to fix and empty hoops to prepare for the next crop. Combine all of that with the need to scout crops and it’s more than enough to make a person’s head spin!

Here’s what I’ve seen on my recent visits to greenhouses.


Great job, everyone! Most crops in most greenhouses look healthy right now. There were, however, a few common correctable conditions observed over the past couple of weeks.

Iron deficiency

The most common symptom of iron deficiency is interveinal chlorosis in the newest leaves. It has the potential to show up in any operation that uses water containing substantial amounts of calcium carbonate (high alkalinity). Here in Michigan, that means just about everyone.

High alkalinity water can raise the pH of soilless substrate over time which, in turn, reduces the availability of iron (and several other nutrients) in the root zone. Some species are more sensitive to these conditions than others and have been designated as “iron-inefficient” plants (e.g., Petunia, Calibrachoa, bacopa, Vinca and others). These are going to be your indicator species and they’ll be the first to display symptoms.

Low fertility

Another minor issue I’ve seen in several greenhouses is sub-optimal fertility levels in some crops. One of the primary symptoms is lower leaf yellowing due to nitrogen deficiency. In time, the whole plant can show symptoms, appearing light green or even yellow. I’ve seen a couple cases recently where low substrate fertility, measured as electrical conductivity (EC), in combination with high substrate pH presented some unusual symptoms due to several simultaneous micronutrient deficiencies.

Causes for low electrical conductivity are often easy to diagnose. It may be as simple as a malfunctioning injector (see the “Calibrating the Injector” section in “Fertilizer Injectors: Selection, Maintenance and Calibration” by Bodie Pennisi and Raymond Kessler, University of Georgia Extension). Also, check if the crop is receiving more water than it needs. When a soilless substrate looks dark brown or black, it’s an indication the media is overly wet. Additional irrigations while the substrate is already saturated will leach nutrients, suffocate the roots and set the stage for a number of physiological disorders and root zone pathogens.

For more detailed discussion and additional tips, check out these articles on iron deficiency, iron-inefficient species, and substrate pH and electrical conductivity management.

Contact a member of the Michigan State University Extension floriculture team for more details and technical assistance.


Bacterial and fungal disease pressure remains low in southwest Michigan. In my trips through greenhouses, I have seen very few disease symptoms. Circumstances seem similar in other parts of the state as scouting reports have very little to say on the subject. However, I have come across stunted plants and symptoms of root rot in some propagation houses and production ranges. In nearly every situation, the flats were being hand-watered and the substrate was rather soggy.

It’s a tough task to obtain desirable moisture levels in every crop this time of year. A one-size-fits-all irrigation plan doesn’t work very well. Moisture requirements vary among plant species and growth stages—and that’s before we consider the variation in environmental conditions between rows, bays, ranges and even geographic locations.

Low disease pressure is good news for growers, as almost any early-season plant material should have shown some signs of infection by now. If you do think something might be infecting your plants, don’t hesitate to contact your local Extension educator or outreach specialist.


Similarly, insect pests haven’t been much of a problem at any of the places I’ve visited lately. Thrips have been well-controlled in most greenhouses. We have had about 10 sunny days in a row in southwest Michigan and I anticipate an increase in thrips activity as a result of consistently warmer temperatures.

Keep an eye out for a significant spike in weekly sticky trap counts. If this happens, Dave Smitley from the MSU Department of Entomology recommends increasing the spray application frequency to twice a week (or no more than five days apart) until weekly trap counts are under 10 thrips. In this situation, rotate the mode of action every two to three applications to avoid insecticide resistance.

Broad mites are still being found on some of the most commonly infested plant species. We’re at the point in the season where symptoms should start becoming very apparent. Take some time to examine the meristems on Torenia, Scaevola, Thunbergia, Reiger begonias and Verbena for brittle and distorted tissue. Interestingly, I haven’t come across an infestation on New Guinea Impatiens yet, which is a departure from recent seasons.

Broad mite damage can sometimes resemble the symptoms of excess soluble salts in the substrate, especially in New Guinea impatiens and begonias. Contact your local Extension educator or outreach specialist if you spot something odd happening with new growth. The quicker we can catch it, the better.

Aphids are starting to be found in greenhouses across southwest and west central Michigan. While most of our problematic aphid species will feed and reproduce on a wide variety of plant species, they still show distinct preferences. Up to this point, we have been finding them primarily on Calibrachoa, Ipomoea, tomato, eggplant and pepper. Even though they’re larger than many other pests, they are adept at blending in with their environment. Keep an eye out for their white “skins” and sticky “honeydew” excretions on plant foliage.

Conventional management product suggestions for all three pests can be found in the MSU Extension’s Greenhouse Pest Management with Insecticides: 2018 Recommendations.

Thanks to Mark Crossley of West Michigan IPM for sharing his time and expertise.

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