Matthew Blanchard and Roberto Lopez
Horticulture

In the previous Greenhouse Alert, we discussed several environmental strategies for height control of greenhouse crops. In this issue, we focus on a few cultural strategies that can be used to control plant height. In most situations, experienced growers will utilize a mix of cultural, environmental and chemical techniques to produce crops within their height specifications.

Cultivar selection
The first step in controlling plant height begins with selecting the cultivars that meet your needs. Each year we are inundated with catalogues and advertisements showcasing the newest genetics. In addition to flower characteristics and disease or insect resistance, breeders are also selecting new plants that have a compact growing habit. Remember to consider this attribute when choosing a new cultivar.

Plant nutrition
Fertility can also influence stem extension and plant height. It is easier to produce compact finished plants when starting with compact plugs. This can be accomplished by limiting the amount of fertilizer (particularly nitrogen and phosphorus) applied to young plants. To learn more about plug fertility, check out an article posted on the North Carolina State University website: http://www.ces.ncsu.edu/depts/hort/floriculture/plugs/plugnut.pdf

Another nutrient strategy to help control plant height is to use a fertilizer that primarily contains the nitrate form of nitrogen. In commercial fertilizers, nitrogen is available in different forms including ammonium, nitrate and urea. When selecting a fertilizer, keep in mind that ammonium and urea forms of nitrogen can promote elongated growth that may produce more leggy plants.

As with plugs, limiting the amount of phosphorus can inhibit stem extension after transplant. Application of a fertilizer solution containing 10 to 15 ppm of phosphorus at every irrigation is sufficient to meet the needs of most plants without excessive elongation. Be careful not to take this to an extreme; if plants are not provided with any phosphorus, they may show deficiency symptoms such as overly stunted growth and purplish leaves.

Water stress
Limiting water availability to plants can be a useful tool for controlling plant height. A plant subjected to water stress will have a slower rate of growth and stem elongation. When using this technique to control plant height, be careful not to let your crop become too wilted. An experienced grower will know how much water stress their crop can tolerate before reaching the permanent wilting point, which is when plant damage occurs. Also, some greenhouse crops recover from wilting much better than other crops.

Cold water on shoot tip
Recent research at the University of G uelph has shown that the use of cold water can be very effective in controlling plant height when applied to the growing tip of Easter lilies. Unfortunately, most bedding plants studied have not shown a similar response to cold water. The only responsive species were begonia, impatiens, sweet potato vine, and zonal geranium. For more information on using cold water to reduce stem extension, visit: http://www.canadiangreenhouseconference.com/attendees/2003/Blom2.htm

Dean Krauskopf
MSU Greenhouse Educator, Southeast Michigan

There has been a report of downy mildew on cutting propagated coleus. If you’re growing these very popular plants, Dr. Mary Hausbeck, MSU greenhouse plant pathologist, has summarized what you need to know about how to identify the disease and prevent or control an outbreak.

Downy mildew checklist

• Yellowish or pale green foliage
• Downward curling of the leaves
• Distortion of the leaves
• White to light gray fuzz on the undersides of the leaves
• Emerging leaves that are small and/or discolored (yellow or pale green)
• Plants may be stunted

The most obvious sign of downy mildew is the white, grayish fuzz that develops on the underside of the leaves. This can go unnoticed because growers are not accustomed to turning over the leaves to look for plant problems. Perhaps other than an especially severe case of Botrytis, no other disease causes such obvious gray fuzz, especially on the underside of the leaf. This disease should not be confused with powdery mildew, which affects some floriculture crops and has white fuzzy growth.

When scouting coleus, examine fully expanded leaves on one out of every 30 plants, paying special attention to the undersides of leaves. Plants may be infected with downy mildew but not show white or grayish fuzz immediately, so scout at least weekly. Diseased plants should be discarded immediately but not placed in a cull or compost pile. Healthy-appearing plants adjacent to the diseased plants should also be discarded. All remaining coleus anywhere on the premises should be treated with fungicides.

Fungicides should be applied preventively before disease develops. Downy mildew has the bad habit of changing and becoming resistant to the systemic fungicides (example: Subdue MAXX). Since Subdue MAXX is especially at risk it should only be used once in a production program and must only be applied as a drench.

Tom Dudek
District Extension Horticulture and Marketing Educator

In the past week or so I have been called out to a number of greenhouses to look over various plant problems. In walking through the houses, I have noticed that Botrytis blight has been present on either the foliage or the flower blossoms of annuals and perennials. With the forecast indicating some portions of the state may see cloudy-rainy weather for a few days its important to review your strategies for managing Botrytis.

Earlier this year Dr. Mary Hausbeck plant pathologist at MSU authored an excellent article on the disease and covered the cultural and chemical strategies one needs to use to reduce the problem and insure that the plants you ship out are healthy.

Go to: www.ipm.msu.edu/grnhouse05/G10-11-05.htm#8 or review the October 25, 2005 Issue #11 of the 2005 Greenhouse Alert for all the details.

When you do, follow her sanitation recommendations about taking the dead plant debris off the plants and benches. Be sure to put it in the trash. Remove the trash barrels from the growing range and deposit the trash in an outside dumpster. Leaving Botrytis infested plant debris near the greenhouse area is asking for a reinfestation!

Tom Dudek
District Extension Horticulture and Marketing Educator

The following tips will help nursery growers deal with frost-damaged plants. If you have specific questions, feel free to contact your local MSU Extension horticulture educator.

Many perennials in the field and in containers that were not protected from temperatures that reached the mid to upper 20’s were frost injured from this week’s cold snap across the state and will be showing the effects in the next few days. Most perennials will re-flush new leaves if the existing foliage was killed back. Daylily ferns and hosta, and others will flush out new leaves but at a slower pace than the initial flush and it may take three to four weeks under normal spring weather conditions.

Day length continues to increase for the next 8 weeks helping leaf expansion, so that will be in your favor.

For other plants lightly damaged by frost, consider a light pruning or pinching back to remove the damaged tissue. Use a fungicide to reduce the risk of Botrytis blight or a bacterial infection. (See above-mentioned bulletin).

If irrigation for frost protection was excessive, you may have leached out your fertilizer from the media or the soil. Use a solubridge to check your EC levels and consider reapplying fertilizer if necessary.

Extra irrigation for frost protection can also increase your risk for Pythium root rot. Check root systems and if needed apply a fungicide like Truban or Subdue to protect roots against Pythium.