Purdue News

January 9, 1998

Perennial plants at risk, but insects not affected by warm spell

Acting state climatologist Ken Scheeringa said January thaws are a regular occurrence, but the recent warm spell was on the extreme side.

"We saw a steady climb to the 50s, and now a downward slide in the 40s and then to the 20s," Scheeringa said. "This instance is rare. We've had thaws before, but not like this."

According to Purdue Extension horticulture specialist Rosie Lerner, early flowering bulbs will have the most dramatic response. Plants such as tulips, daffodils and crocus may be showing a bit of foliage above ground, especially if planted near buildings or other warm, protected locations, she said.

Those leaves may be nipped back when temperatures drop below freezing again, but in most cases, the flower buds should still be protected inside the bulb below ground. If flower buds have broken out, they too could be nipped by a hard freeze, but the bulbs themselves will survive and come back next year, even if the flowers don't make it this year, Lerner said.

There's not much a gardener can do to prevent this turn of events except let nature take its course. Mulching over the plants now might smother them and would only serve to warm the soil further, causing continued growth, she said.

The mild temperatures should not cause much damage to dormant fruit and ornamental plants either, said Bruce Bordelon, associate professor of horticulture. In general most perennial plants need between 800 and 1,000 hours of cold temperatures before they would break dormancy. "We have not accumulated enough 'chilling hours' for crops and plants to break dormancy even if the weather is warm," Bordelon said. "It's like they won't wake up if they haven't had enough sleep."

Cold hardiness, or the lack thereof, is a greater concern, he said. At this time of year, plants usually are at their maximum level of cold hardiness and are capable of surviving temperatures below zero. However, exposure to several days of warm temperatures can cause plants to lose that protection. Plants will recover cold hardiness as temperatures gradually return to normal, but the plants could suffer cold injury if temperatures fall too quickly, Bordelon said.

No matter how warm or cold it is this time of year, said entomology Professor Tom Turpin, the insects are here to stay.

"People are hoping against hope that something will happen in winter and the bugs won't be here this year," Turpin said. "With the majority of pests, you don't really know, but major pests adapt to winter."

One false hope, he said, is that the warm temperatures will drive insects out of hibernation, and then they'll die off when the cold weather returns.

"Insects are cold blooded and can't function in winter. But a lot of insects survive the cold through hibernation, such as caterpillars in cocoons, butterflies in chrysalis and eggs in the soil," Turpin said. "They've been around for three hundred million years, and they'll continue to be here."

CONTACTS: Bordelon, (765) 494-8212; Turpin, (765) 494-4568; Scheeringa, (765) 494-8105; Lerner, (765) 494-1311

Compiled by Chris Sigurdson, (765) 494-8415; home (765) 497-2433; E-mail, sig@ecn.purdue.edu
Purdue News Service: (765) 494-2096; e-mail, purduenews@purdue.edu

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