Measuring dormancy and cold tolerance

Knowing when plants will come out of dormancy is tricky when the weather is unpredictable.

With fruit, there are two types of winter dormancy, said Mark Longstroth, an Extension educator with Michigan State University. With endo-dormancy, plants will not grow even if the conditions are warm enough for growth. With eco-dormancy, growth doesn’t occur because conditions are too cold.

Endo-dormancy and the plant’s chilling requirements normally prevent growth from occurring during winter warm spells, according to Longstroth.

If growth begins, injury can happen, as the plant cannot increase its ability to withstand the change. Endo-dormant plants in the deep winter can become cold hardy up to -20˚ F or below, Longstroth said. But once growth begins, they lose the ability to withstand extreme cold.

There is one very easy way to know if endo-dormancy has been completed, he said.

“You can take cuttings from the plants you are interested in and bring them inside to see if they will grow under warmer conditions,” he said. “Collect several healthy shoots from the plants and place them in a vase or glass of water to provide consistent warm temperatures.”

If the buds begin to swell and grow within a week to 10 days, the plants have enough chilling hours and will begin growth in warm weather, according to Longstroth.

You may need to do this several weeks in a row.

“I usually start collecting shoots in early to mid-January, and generally see growth here in southwest Michigan in mid- to late January,” he said. “Often, the first time I see growth, the growth is ragged and not all the buds break and start to grow. This indicates that not all the buds have completed their chilling requirement, but several have and are ready to grow.”

Once the plants have completed chilling and have moved into eco-dormancy, growth depends on how warm it gets, according to Longstroth.

“If we get warm temperatures and growth begins, then the temperatures falling into the teens or below might cause damage,” he said. “If you force some cuttings, you will have a good idea of how worried about winter cold you need to be.”

By Derrek Sigler