Napa County
Newspaper Articles  Dormant Season

By Val Whitmyre, U. C. Master Gardener

 

The first hint of winter swept through Napa Valley in early November, stripping trees of their fall colors, leaving a patchwork carpet of brilliant leaves. Deciduous plants will continue to drop their leaves this month until only bare stems and branches remain for the duration of winter.

In nature, there is a reason for everything. Trees drop their leaves because they don’t want them to grow during winter. Bare branches may not appeal to us, but when winter sunlight is scarce, these branches allow more sunlight into our homes.

Plants cannot continue to grow indefinitely. If they did, they wouldn’t last long. They must be protected from harsh winter conditions in order to survive. This built-in protection is called dormancy. Dormancy is a physiological condition or mechanism that helps insure the continuation of a species.

Our wonderful fall colors are a signal that plants are going into dormancy. Light and heat are necessary for plant growth. As the days get shorter, plants receive less sunlight each day. Although plants may appear to be dying, they are healthy and will remain so if allowed to remain dormant for winter.

Fertilizing plants with nitrogen, which encourages new growth, is not recommended now because plants need to rest. If buds continue to produce new tender shoots, the shoots would surely be damaged by winter frost and the plant could die. The plant will survive without fertilizer because it has stored carbohydrates to maintain low-level respiration during dormancy. Nor do plants need water during dormancy unless prolonged winds or an unexpected dry spell occurs.

Ornamental, fruit and nut trees each have unique dormant chilling requirements before they may resume growth. Depending on the variety, apple and sweet cherry trees need from 500 to more than 800 hours of temperatures below 45ºF. Nothing will break their dormancy until those hours are satisfied. Forsythia doesn’t need much winter chill and is one of the first shrubs to bloom in the spring. The length of dormancy may vary because winter weather varies each year.

Apples and cherries grow better in cold winter climates that insure their dormancy requirements are met. Pecans, almonds, figs and olive trees grow better in warmer climates because they don’t need much winter chill.   

Seeds have two kinds of dormancy. Some seeds have impervious coats that prohibit the penetration of water, thereby preventing germination, until conditions are favorable. You can nick the seed coat or scrape it with sandpaper or a file to break dormancy and induce germination. This process is called scarification. The warmth of a bird’s stomach or the heat of a forest fire may also break the dormancy of these seeds.  

Other seeds exhibit what is known as embryo dormancy. They require a chilling period to break dormancy. Just as with the fruit and nut trees, each species has its own chilling requirements. In the mountains, wild sweet pea seeds that spend a whole winter below several feet of snow will readily germinate when exposed to sunlight and a warming soil the following spring. 

You can assist the process of breaking embryo seed dormancy by putting seeds in moist paper towels or moist potting soil and keeping them in the refrigerator for several weeks.

Bulbs such as onions and tulips benefit from some chill time. After harvest, commercial growers keep onions in near-freezing conditions to lower their respiration rate and help retard spoilage. Because tulips favor cold winters, you should refrigerate bulbs for six weeks before you plant them to prevent short stems and flowers that bloom at ground level. 

If you really want to help your plants now, rake all the debris and diseased plant material away from them. Put this into a recycling bin, not your compost pile, then spread a three-inch-thick mulch of compost or aged manure under the plants. You could use a 0-10-10 fertilizer now to encourage spring flowers and root growth. Right now, your plants are saying, “Just leave us alone. We know what we’re doing.” 

 
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