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Wednesday, October 4, 2023
Oct. 4, 2023

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Gardening with Allen: Light is a plant’s wake-up call


In various articles you have described plants as long-day, short-day and day-neutral. Was that a reference to the number of hours of sunlight? How do plants respond to changes in day length?

Day length or photoperiod is a major factor used by plants for various processes. Plants use day length to tell them when it is time to go dormant in the fall and when to wake up from dormancy in the spring.

Most of us assume it is the change in temperature that induces dormancy. But changing temperature is variable and is not always accurate enough to protect plants from sudden temperature drops in the fall. Likewise, an unusually warm winter spell could stimulate new growth that would be damaged by a return to normal cold. I noticed that trees growing here started losing leaves at about the same time as the same tree species in a colder winter climate.

Flower formation and subsequent fruit and seed development is another major growth process controlled by day length. Most of the spring- and summer-blooming flowers that we grow in our gardens are long-day plants. They do not all bloom at the same time. Each variety is triggered to start flower initiation at a specific day length and often continue blooming until the shorter day length is reached again.

Fall-blooming flowers like chrysanthemums are short-day plants and start flower initiation when the specific shorter day length is reached. Greenhouse growers use black cloth or lights so they can force chrysanthemums to bloom any time of year.

The development of day-neutral strawberry varieties for the California strawberry industry is an interesting story. Royce Bringhurst, a plant breeding professor at UC Davis, became aware of wild strawberries growing in the Utah mountains that bore fruit all summer. This was an indication of their day-neutral status. Up until that time, all garden and commercial strawberry varieties were long-day plants that bore fruit for a maximum of six weeks in late June and July.

Bringhurst crossed the wild Utah plants with commercial varieties and selected for commercial size and quality and the day-neutral character. These varieties produced fruit for the full season in coastal California. They also produce from summer until fall in Washington and Oregon. The University of California strawberry variety sold locally is Seascape.

Most perennial flowers are long-day plants with a limited blooming period. However, there are a few perennials that are naturally day-neutral and have much longer bloom periods. Three of my favorites are geranium Rozanne, coreopsis Moonbeam and Peruvian lily (Alstroemeria).

Green Fuse Botanicals has a breeding program to develop day-neutral perennial flowers to extend their blooming season.

One of the first to be introduced is the All-America Selections award-winning ground cover Shasta daisy Carpet Angel.

Green Fuse does not sell directly to gardeners, only to wholesale greenhouse growers who in turn sell them to retailers. These new day-neutral perennial varieties will be branded with the name First Light. I expect to see some of them in full-service retail nurseries and garden stores soon.