Friday, June 26, 2009

A little time to chill out is good

A reader (!) had this question about the effects of 2008's cold December weather on the garden:

"Do you have any information about how a longer cold weather spell (December) caused plants (and weeds) to grow like they are on steroids? My clematis are over 12 feet tall this year (no fertilizing or gardening changes) when they are normally 6-8 feet tall. In addition, I have10-15 stems up for one plant when there are usually 4-5. These are on plants that are 5-7 years old. Other plants are showing the same energy. Might this cause them to wear themselves out before the end of their growing period later this summer?"

Linda Beutler, curator of the Rogerson Clematis Collection and author of Gardening with Clematis (Timber Press), had this to say:
"Most clematis we grow in the Pacific Northwest, with the exception of the evergreen types, are much hardier than Zone 8. Many will not bat an eye at a Zone 4 winter. Here in Zone 8, they don't get as much winter rest (vernalization) as they might like, meaning they are in active growth into true winter, and wake up earlier, with new basal shoots and leaf buds on old wood in evidence by Valentine's Day or earlier. This year the climate was more to their liking, hence the lush growth and record-setting blossom diameters. No, they will not wear themselves out! Although... a bit of thinning, and deadheading what you can reach, would be appreciated, but not necessary."

Clematis fan? Check out The Rogerson Clematis Collection, located at Luscher Farm in Lake Oswego, is one of the largest in the Western hemisphere. It includes 500 taxa and almost 900 individual plants. Watch for Linda's guided tours of the collection during Le Tour des Plants, starting September 12.

Vernalization is another way to say that the plant needs prolonged exposure to the cold of a typical winter (expressed in hours at 45ยบ or less) in order to bloom. Plants grow, flower, set seed, and then go to sleep. In a way, winter chill is the alarm clock that tells the plant to wake up and make flowers when the weather warms. Without the cold, the plants are like me without coffee: groggy and not so productive. Certain plants evolved this way to ensure they would flower at the right time of year to be in synch with their preferred pollinator or to avoid adverse weather during bloom.

Some plants will not flower at all until they get the proper cold exposure. Other plants will flower eventually without vernalization, but will flower later and show fewer flowers spread out over time. If you are not getting much fruit from your fruit tree, lack of winter chill may be a problem. Check with your Extension agent or Master Gardener by browsing to or

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