Floriculture Greenhouse Update

Heat Delay - Garden Mums

Thursday, 02 September 2010 11:41

High night temperatures can cause mum flowering to be delayed.

Chrysanthemums are short-day plants. Both flower initiation and development of the flower buds occur more rapidly under short days than under long days. However, temperature has a greater influence than daylength on flowering of garden mums.

Delay of flowering and in more extreme cases, failure of buds to develop properly (crown buds) are often associated with heat delay. Heat delay occurs when night temperatures are greater than 73F. Garden mum cultivars vary in their sensitivity to heat delay.  Also research at Kansas State University has shown that plants are more sensitive to heat delay when plants are subjected to high night temperatures during the first 2 weeks of short days, during flower bud initiation. Plants subjected to high night temperatures after 3 weeks of short days showed no permanent damage and only exhibited delayed flowering.

Dr. Kimberly Williams, Kansas State University explains heat delay in this short video.

Tina Smith, UMass Extension and Leanne Pundt, UConn Cooperative Extension

Reducing Storm Damage to Your Greenhouses

Thursday, 02 September 2010 08:45

Nature seems to be getting more violent in recent years with frequent earthquakes, increased numbers of hurricanes and record breaking snowstorms.  Insurance damage claims have increased considerably.  The International Building Code has revised upward its wind and snow loading requirements for some areas of the U.S.  

Each year there are reports of greenhouses that have been damaged by weather and natural events.  Greenhouse design is different than conventional farm buildings in that the structural profile has to be small to allow maximum light to reach the plants. Most farm buildings are over designed to handle severe weather conditions.

Storm damage to greenhouses can include racking of the frame, bending of the hoops, broken glass or torn plastic and uplifted foundation posts.  Preparation ahead of time can minimize the damage.

Wind loading
Wind forces that act on a greenhouse are influenced by numerous factors including the basics wind speed, building orientation, exposure, height and shape of doors or vents that may be open.  The wind passing over a greenhouse creates a positive pressure on the windward side and a negative pressure on the leeward side.  These can combine to create a force that wants to collapse or overturn the building.  An 80 mph wind can produce a pressure of 16 pounds per square foot (psf).  For example, the 10' by 100' sidewall of a gutter-connected greenhouse would have to resist a 16,000 pound force.

Wind can also create a force similar to an aircraft wing that wants to lift the greenhouse off the ground.  An 80 mph wind blowing perpendicular to the side of a 28' x 100' hoophouse can create a lifting force of 220 pounds per foot of length or 22,000 pounds of uplift on the whole structure.  When you consider the total weight of materials and equipment in the greenhouse is about 6000 pounds, the foundation must have a withdrawl resistance of about 300 pounds each.  This is why building inspectors frequently require that the posts be surrounded by concrete.

Although you have no control over the force or direction of severe winds, here are a few tips to help minimize storm damage:

• Check the area for loose objects.  Anything that can be picked up and hurled through the glazing should be secured or moved indoors.  Metal chimney (stove pipe) sections should be secured with sheet metal screws.
•  Inspect for dry or weak tree limbs that could fall on the greenhouse.
•  Close all openings including vents, louvers and doors.  The effective force of the wind is doubled when it is allowed inside the building.  The wind on the outside puts a pressure or lifting force on the structure.  The wind inside tries to force the walls and roof off.
•  On air inflated greenhouses, increase the inflation pressure slightly by opening the blower’s intake valve.  This will reduce the rippling effect.  Check to see that the plastic is attached securely and that any holes are taped.
•  Disconnect the arm to the motor on all ventilation – intake shutters and tape the shutters closed.  Then turn on enough exhaust fans to create a vacuum in the greenhouse.  This will suck the plastic tight against the frame.
•  Windbreaks can reduce the wind speed and deflect it over the greenhouse.  Conifer trees (hemlock, spruce, pine, etc.) in a double row located at least 50' upwind from the greenhouse can reduce the damaging effects of the wind.  Wood or plastic storm fencing can be used as a temporary measure.

John Bartok, Extension Professor Emeritus and Ag Engineer, UConn

Fact Sheet: Reducing Storm Damage to Your Greenhouses (includes snow loading also)

Note: With wind, rain and cooler temperatures, growers are advised to continue inspecting plants and use preventative sprays for diseases such as Downy mildew on basil and Chrysanthemum white rust on garden mums.  Leanne Pundt, UConn

Basil - Downy mildew

Tuesday, 17 August 2010 10:16

This information was summarized from a recent article by Rob Wick and M. Bess Dicklow, Plant, Soil and Insect Sciences, UMass Extension published in Floral Notes, 23(1), July-August 2010

During 2008 and 2009, Downy mildew (Peronospora species) of basil occurred throughout the east coast in epidemic proportions both in the field and in greenhouses. Although the downy mildew pathogen has been detected in basil seed; seed transmission is probably a rare event. Air-borne dissemination from infected plants is more likely.

Symptoms on the top of the foliage occur as yellowing as if the plants are nitrogen or nutrient deficient. When spores are produced, a characteristic gray, fuzzy growth on the underside of the leaves is evident. Symptoms of downy mildew on basil can easily be mistaken for nutritional deficiency.

The most important environmental factors favoring disease development are high humidity and extended leaf wetness. Cultural controls are directed at reducing the amount of time leaves remain wet by reducing plant density and watering early in the day. In the greenhouse, horizontal air flow can be increased and condensation reduced by fans. Removing and destroying severely infected plants can slow disease development.
Field trials conducted in southern New Jersey in 2009 determined that commonly-grown sweet basil (Ocimum basilicum) cultivars such as ‘Poppy Joe’ and ‘Nufar’ were the most susceptible to downy mildew. The least susceptible basils include the lemon and spice types such as O.x citriodorum and O. americanum  cultivars, “Lemon Std’, ‘Lemon’, ‘Lime’, ‘Spice’, ‘Blue Spice’ and ‘Blue Spice Fil’.

Few fungicides are labeled for herb plants. Copper products and phosphites are labeled for use on basil. It is the grower’s responsibility to read and follow label directions. The label is the law and any recommendations made here are superseded by the label.

At the University of Massachusetts, we are investigating methods to control this disease with biological control agents. We are interested in collecting live, infected plants.

If you think your basil plants are infected, please call or email Rob Wick, Dept. of Plant Soil and Insect Sciences; 413-545-1045, This e-mail address is being protected from spambots. You need JavaScript enabled to view it .

Photos: DM symptoms on top of Basil foliage, DM symptoms on underside of leaf

Robert Wick and Bess Dicklow, UMass Extension

Note: Downy mildew has also been observed recently on ornamentals in the Lamiaceae family, which includes basils (Ocimum spp), mints (Menta spp.), sages (Salvia spp) and other aromatics. According to Margaret McGrath, Cornell University, the coleus and basil downy mildew pathogens have now been demonstrated to be genetically different; therefore, these ornamental plants are not considered potential alternative host. However, there are many ornamental basils that are also hosts to pathogen affecting basil grown for use as an herb.
Tina Smith, UMass Extension and Leanne Pundt, UConn Extension

Fact Sheet with photos and more details: Basil Downy Mildew (Cornell University)