Why some plants are “fooled” by a warm December and some aren’t

Here in Michigan – and, it seems, most of the Eastern US – we’ve been having unseasonably warm weather and there are odd things afoot in the garden. Some plants that would normally be dormant coming back into growth. But perhaps odder is that while some plants have been “fooled” by the unseasonable heat, others are still resolutely dormant and not pushing any growth at all despite the warmth. Why is that?

A wild rose with buds still tightly dormant despite unseasonable warmth
A wild rose with buds still tightly dormant despite unseasonable warmth
A 'Knock Out' rose pushing growth during a warm spell.
A ‘Knock Out’ rose pushing growth during a warm spell.

There are a lot of factors that determine when a plant is dormant and when in active growth, a key one in this context is whether they have a vernalization requirement or not. In simple terms, some plants, once they go dormant for the winter, will refuse to come back into growth until they’ve experienced a period of cold temperatures. Once they’ve been through that cold, the plant is termed to be vernalized and will then burst into vigorous growth as soon as the weather warms up again.

You’ve probably run up against a vernalization requirement in terms of bulbs like tulips. That requirement is why you need to give tulip bulbs a cold treatment in order to force them to bloom indoors, and why southern gardeners without sufficient natural winter cold have to pre-chill their tulips in order for them to bloom. The adaptive advantage of this is obvious in a year like this, as it prevents plants from jumping the gun in a mild December and getting damaged by the real cold when it arrives.

So why do are some plants lack this adaptation and come into growth in a freak warm spell? Some are adapted to life warmer climates and sometimes it is the work of humans. Modern hybrid roses, for example, have had their vernalization requirement bred out of them. The downside is that this makes them more susceptible for winter damage sometimes, but the plus side is that it is part of what causes them to bloom all summer long rather than just once in the spring the way most of their wild ancestors do.

Published by

Linda Chalker-Scott

Dr. Linda Chalker-Scott has a Ph.D. in Horticulture from Oregon State University and is an ISA certified arborist and an ASCA consulting arborist. She is WSU’s Extension Urban Horticulturist and an Associate Professor in the Department of Horticulture, and holds two affiliate associate professor positions at University of Washington. She conducts research in applied plant and soil sciences, publishing the results in scientific articles and university Extension fact sheets. Linda also is the award-winning author of five books: the horticultural myth-busting The Informed Gardener (2008) and The Informed Gardener Blooms Again (2010) from the University of Washington Press and Sustainable Landscapes and Gardens: Good Science – Practical Application (2009) from GFG Publishing, Inc., and How Plants Work: The Science Behind the Amazing Things Plants Do from Timber Press (2015). Her latest effort is an update of Art Kruckeberg’s Gardening with Native Plants of the Pacific Northwest from UW Press (2019). In 2018 Linda was featured in a video series – The Science of Gardening – produced by The Great Courses. She also is one of the Garden Professors – a group of academic colleagues who educate and entertain through their blog and Facebook pages. Linda’s contribution to gardeners was recognized in 2017 by the Association for Garden Communicators as the first recipient of their Cynthia Westcott Scientific Writing Award. "The Garden Professors" Facebook page - www.facebook.com/TheGardenProfessors "The Garden Professors" Facebook group - www.facebook.com/groups/GardenProfessors Books: http://www.sustainablelandscapesandgardens.com

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