Winter 2025
What are Our Perennials Doing in Winter?
by Cynthia Engers
It’s clear that with the sun low in the sky, shorter days, and cooler temperatures, plant growth has slowed, almost to a stop. Most of the plants are going dormant and will be in full dormancy by January. What is going on? We know they’re not dead, but is anything happening below the soil, if not above it?
Dormancy, which in our area can also occur in summer, is a means for plants to withstand unfavorable conditions, such as drought or cold. The plants enter a period of minimal activity – most of which happens underground.
Preparation for dormancy starts well before winter arrives. When the average temperature falls and the sun moves lower in the sky, hormones such as abscisic acid trigger a slowing of photosynthesis and respiration. Plant enzymes that drive metabolism work poorly in cooler temperatures and growth stops. We all enjoy the beauty of fall color in leaves – but what we primarily see with slowed photosynthesis is the result of the absence of chlorophyll in the leaves which masks carotenoids responsible for yellow and orange colors. The slow growth of the plant also leads to excess sugars in the leaves which produce anthocyanins that create red and orange colors.
"Green, yellow, and orange colors are always present in leaves, but chlorophyll – the green pigment responsible for photosynthesis – is so dominant during most of the year that it masks out the other colors," says Igor Lacan, UC Cooperative Extension Urban Forestry Advisor.
Because plant growth has slowed, any remaining leaves producing carbohydrates via photosynthesis route the carbohydrates to the plant’s storage areas; the stem and especially the roots. These “sinks” will provide the minimal energy for the plant to get through the winter and will provide the energy to resume growth in the spring before the plant has new leaves to generate new carbohydrates and energy. Cool-season vegetable gardeners benefit from this mechanism when they harvest their carrots, beets, and radishes.
But our dormant plants are even more clever than that. Particularly needed in climates colder than ours, perennials also have mechanisms to protect themselves from freezing temperatures. Individual plant cells, especially in roots, accumulate sugars and salts which decrease the freezing point of the cells. In addition, water moves from within plant cells into the spaces around them, thus protecting the actual cells from freezing (though ice crystals may form between cells.)
Do all plants have these mechanisms to survive the winter? No, they don’t. Annuals funnel all their surplus energy into seed production rather than storing it in the roots for the upcoming season. And plants like tomatoes, which are technically perennials in tropical climates, lack the mechanisms to store sufficient salts, sugars, and certain proteins within the cells to protect them from freezing.
Winter months can actually benefit plants that require chill hours
On average, Northern California receives between 800 and 1500 chill hours each winter which is ideal for our famous fruit and nut production. On the other hand, as many home gardeners have discovered, this is not enough chill time for various other plants and bulbs, particularly hybrid tulips, which require up to 2500 chill hours. It also explains why apples, cherries, plums, and peaches grow more readily in Northern California than in Southern California.
What else is happening to our plants in the winter?
Soil microbes are especially active in the fall. However, like the perennial plants themselves, microbes living in the soil generally just slow down in winter. During winter here in Contra Costa County, the soil maintains a more constant temperature than the air and is unlikely to freeze beyond the surface, so our soil denizens can continue their work, albeit at a slower pace. In addition, organic mulch and cover crops provide a sort of blanket for the cool soil, further facilitating microbial activity.
While a complete discussion of the relationship between plant roots and organic matter is beyond the scope of this article, it is pertinent to know that many of the beneficial soil microorganisms associated with plant roots, such as bacteria and fungi, synthesize plant hormones. One of these, abscisic acid noted above, is vital for managing plant dormancy.
In conclusion, it is fair to say that, although our plants may appear inert in the winter, they are still very busy executing their life functions during this season. We just have to look a little deeper to understand how!
References:
Where do fall colors come from? - Green Blog - ANR Blogs
Home Page - Healthy Soils for a Healthy California
California Master Gardener Handbook
University of California Agriculture and Natural Resources
Dormancy: A Key to Winter Survival, University of New Hampshire Master Gardeners
Microbes in Your Soil
By Veda Scherer, University of California Cooperative Extension Master Gardener of Calaveras County