Southwest Michigan greenhouse report — February 2018

Be on the lookout for spider mites and broad mites.

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

Spring crop status

Just a few weeks ago, things were a bit sleepy in the southwest Michigan greenhouse scene. Now that the second week of February has come and gone, things are starting to perk up. Almost everyone has started their spring crop and while the main production areas are mostly empty and cold, seed and propagation houses are busy and full of life.

The smell of wet peat is in the air. Seedlings are emerging and rooted cutting are starting to find their way into baskets and planters. It’s growing time!


Fungal and bacterial disease pressure has been very low in southwest Michigan and it appears to be the same throughout the west central and eastern sides of the state as well. However, Michigan State University Extension greenhouse educators have seen symptoms of root rot on vegetative cuttings in several propagation houses.

Many factors contribute to cutting propagation success including time to stick, light intensity, misting frequency, humidity, root-zone heating, temperature, ethylene exposure and using rooting hormones. When we have observed root rots or damping off, the liners were almost always being hand-watered and the substrate was excessively wet.

A typical propagation house will have many different plant species in various propagation stages, each with different root development rates and moisture demands. This combination of variables can make it difficult to optimize irrigation levels for any single species. Fortunately, W. Garrett Owen has written a timely article on how to manage moisture in vegetative cutting propagation. It’s an excellent discussion of the topic with helpful tips on how to overcome some of these common problems.


Based on sticky traps counts and leaf quality, thrips pressure looks to be low in most places. Because thrips pupae are usually found in the substrate and their eggs are laid inside leaf tissue, both life stages are extremely difficult for propagators to control. Therefore, incoming plant material can be a major source of early-season thrips (assuming good sanitation practices).

Winter temperatures can suppress thrips population growth, so now is the time to be proactive with your thrips management. Consider using an entomopathogenic fungus (e.g., Botanigard, Met-52, etc.) as a low-toxicity sprench for your plugs and liners, especially in those perennially troublesome crops such as Cordyline and Vinca.

Download MSU Extension’s 2018 “Greenhouse Pest Management with Insecticides” tip sheet for additional tips and a complete list of recommended pest management products.

Spider mites have been found on several crops, especially Ipomoea and Cordyline. Broad mites have been found on Thunbergia, Torenia, Agastache and Reiger Begonia. Both pests feed on leaf tissue by piercing plant cells with their mouthparts and sucking out the contents.

Spider mite damage looks like patches of tiny, yellowish-white speckles as if someone took sandpaper to the leaf. Broad mites, on the other hand, have phytotoxic saliva that causes plant tissue to become hardened, brittle and distorted. For comparison, broad mite damage can look remarkably similar to what 2,4-D herbicide damage looks like in the field. More information on broad mites can be found in a recent article by Heidi Lindberg, “Broad mites in ornamental crops – Part 1: Challenges and treatments.”

When scouting your crops, look for broad mite damage on new plant tissue at the apical meristem and spider mite damage on older leaves in the lower canopy. Conventional management product suggestions for both pests can be found in the “Greenhouse Pest Management with Insecticides” tip sheet.

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