We are going to have some cold overnights through the end of the week. Because of that, I wanted to make sure you had the information you need to keep your outdoor plants from frost and freeze.
Much of this information has been provided by Rosie Lerner, Extension Consumer Horticulture Specialist and Master Gardener State Coordinator from Purdue University. Additional information on horticulture can be found at http://www.hort.purdue.edu/ext/conhort.html . This information is also relevant to areas of Northwest Ohio and Southern Lower Michigan where temperatures have been very similar to Northern Indiana.
Following a particularly mild winter but cold enough to easily satisfy the chilling requirements for flowering, our spring blooming plants have had a tremendously early and spectacular display this year. Blooms are about 4 -6 weeks ahead of “normal” this year and bud counts are high. Average temperatures across Indiana have been more like late April to early May than mid March.
Due to the recent stretch of record warmth, it’s a bit like the grand finale of a fireworks display. While the sequence of bloom appears to be staying in order, it’s as if the season is on fast forward bringing nearly everything into bloom all together. Our current cast of characters (March 22) in West Lafayette includes forsythia, daffodil, magnolia, redbud, flowering dogwood, ornamental pear, cherry, peach, plum, tulip, serviceberry, Korean spice viburnum, and crabapple to name a few. Similar blooms and flowering have been seen across northern parts of Indiana, Northwest Ohio and Lower Michigan as well.
Looking at growing degree days (GDD) with a base of 50ºF, as of March 22 in West Lafayette the total stands at 233.5 GDD compared to 45.0 this date last year. Lafayette hit 233 GDD on May 5 in 2011. In the South Bend area, GDD for 2012 were at 205 through March 22nd. Through the same time in 2011, GDD stood at just 9.0. GDD at Fort Wayne are a little lower but still quite high for this time of year. Through March 22nd, Fort Wayne has seen 176.5 GDD compared to just 9.0 last year. Lima OH has seen 183.5 GDD this year compared to 15.5 in 2011 and Benton Harbor MI has seen 173.5 GDD compared to just 11 last year. (GDD information courtesy of weather.com GDD calculator).
The likelihood of hard frost and freeze is still high in the coming weeks. According to the Indiana state Climate office <http://iclimate.org/narrative.asp >, the average date of the last freezing temperature in spring ranges from the second week of April in extreme southwest Indiana to the second week of May in the extreme northeast. Two-thirds of the time they occur within a 20- to 24-day period centered at the mean date. The trend of a later date toward the north is reversed in extreme northwestern Indiana, where the average date is about May 1 near Lake Michigan.
For plants whose primary ornamental feature is flowers, you’ve enjoyed the show thus far and barring any really unusual weather events, the plant itself will not be killed by frost/freeze. However, there certainly could be injury to foliage and young twigs, likewise for herbaceous perennials and hardy annuals. Except for conifers, plants that lose leaves or leaf buds will produce new ones. If buds are injured, but not killed, new leaves may be cupped, crinkled, twisted, curled, wilted or tattered. Though unsightly, most plants will eventually outgrow this type of injury. The Purdue Plant and Pest Diagnostic Laboratory (PPDL) has some good articles showing freeze injury from previous years. www.ppdl.purdue.edu/ppdl/hot10/5-14.html and www.ppdl.purdue.edu/ppdl/weeklypics/Weekly_Picture6-25-01-1.html .
With regard to fruit crops, fruit set and development is susceptible to injury when temperatures drop to near or below freezing. The degree of susceptibility depends on several factors, most notably species, stage of development, and temperature (specific temp as well as duration of exposure.)
At petal fall and fruit set, apples, peach, and tart cherry can be expected to have 10% bud kill at 28ºF, but 90% bud kill at 25ºF. Pears are quite similar with 90% bud kill at 24ºF. Below is a page showing the critical temperatures and stages of flower/fruit development. More information on fruit bud hardiness can be found from Michigan State University at http://www.hrt.msu.edu/faculty/langg/Fruit_Bud_Hardiness.html .
Strawberries and grapes are even more sensitive; critical temperature is 30ºF for strawberries. Grapes are a bit more complicated but suffice it to say that once the leaves begin to expand, 28ºF is likely to cause significant reduction in fruit set. More information on Frost/Freeze and impacts on plants can be found at http://www.ces.purdue.edu/extmedia/ho/ho-203.html .
What can home growers do if frost is predicted? Small fruit plants can be covered to provide a few degrees of protection temporarily. Blankets, throws, and tarps can be used but provide stakes, wires, or other supports to keep the weight of the cover off of the plants. Straw would be useful for covering low growing plants such as strawberries and herbaceous perennials. Covers need to be removed as soon as possible after the threat is past to avoid over-heating and over-shading. It is impractical to do much for trees, large shrubs, and large garden areas.
It is important to monitor weather forecasts daily as conditions can change quickly. A temperature change of just a few degrees can have a large impact on whether we see a light frost or a hard freeze. Follow http://www.weather.gov/iwx for all your local weather information, forecasts, advisories and warnings.
April 11th, 2012 at 9:04 pm by Doug Moats under Weather
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