PPDL Picture of the Week for
March 26, 2012

Spring on Steroids!?

Rosie Lerner, Extension Consumer Horticulture Specialist

We are certainly having an amazing spring flowering season, nearly over before spring officially began! Many are wondering whether their plants are going to be in trouble if the temperature drops and what to do to protect them.

Following a paricularly mild winter but cold enough to easily satisfy the chilling requirements for flowering, our spring blooming plants have a tremendously early and spectacular display this year. Blooms are about 4 -6 weeks ahead of “normal” this year and bud counts are high.

Due to the unseasonably high temperatures, 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. I’m sure you can name some others.

Looking at growing degree days (GDD) with a base of 50ºF, as of March 22 in West Lafayette we are at 233.5 GDD compared to 45.0 this date last year. We hit 233 GDD on May 5 in 2011. Weather.com has a GDD calculator where you can enter a zip code and specific date ranges if you want to get data for your locale.

The likelihood of hard frost and freeze is still high in the coming weeks. According to the state Climate office, 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, this most plants will eventually outgrow this type of injury. The PPDL has some good articles showing freeze injury from previous years. Symptoms of Late Frost Injury and Spring Freeze Injury.

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. You can read more about this at "Freeze damage depends on tree fruit stage of development". 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.

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.

So, we have a bunch more weather to get through before we will know whether this is the best season ever, the worst ever, or somewhere in the middle. Meanwhile, enjoy the show while you can!

Click image to enlarge

Weeping cherry and redbud in bloom March 24 2012, cherry already at petal fall!

Magnolia soulangeana March 20, 2012, all petals dropped by March 26. 

Hawthorne (white foreground), redbud (purple left background, crabapple (pink background), rhododendron (right background), March 26, 2012

Tulips, March 26, 2012

Purdue Plant & Pest Diagnostic Lab Purdue Cooperative Extension Service