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How plants react to longest day of the year

Chrysanthemums often are grown in greenhouses so they can be sold in bloom. (Photo: Debbie Arrington)

Some flowers need to count down to bloom time

Happy first day of summer!

The longest day of the year represents a seasonal milestone to people. To some plants, it represents much more.

Many different plants take their blooming cues from the ratio of daylight to darkness. They’re often referred to as “long-day plants” or “short-day plants,” but it’s actually the hours of dark night that cause their reaction.

Called photoperiodism, this reaction can be found in several crops and popular flowers. (A photoperiod is the number of hours of light in a day.) It not only controls flowering but vegetative growth and root development.

To take advantage of that light-sensitive reaction, commercial nurseries often manipulate the number of hours plants are exposed to bright light or total darkness in greenhouses so plants can be shipped to customers in bloom.

Rudbeckia is a long-day plant. (Photo: Kathy Morrison)
Long-day plants naturally bloom in spring as days get longer. Their trigger is when daylight (or darkness) reaches a certain critical point, which varies by species. That group includes pansies, peas, snapdragons, rudbeckias, petunias, coreopsis, lobelias and Shasta daisies.

Many fall- and winter-blooming plants need shorter days and longer nights to trigger their flower cycles. That group includes dahlias, chrysanthemums and poinsettias.

These short-day plants need total darkness through the night to cue their flower cycles. Outdoor or other artificial lighting can interrupt their “sleep” and keep them from blooming.

Conversely, some long-day plants could be forced into bloom with four hours of intense light in the middle of the night in addition to natural daylight hours.

Research at Michigan State University found that dahlias bloomed much faster when exposed to 10 hours of light daily, compared to 12, 14 or 16 hours. When exposed to light 24 hours a day, they didn’t bloom at all.

When will our days and nights be even? That’s the autumnal equinox, which this year falls on Sept. 23.


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Dig In: Garden Checklist

For week of March 26:

Sacramento can expect another inch of rain from this latest storm. Leave the sprinklers off at least another week. Temps will dip down into the low 30s early in the week, so avoid planting tender seedlings (such as tomatoes). Concentrate on these tasks before or after this week’s rain:

* Fertilize roses, annual flowers and berries as spring growth begins to appear.

* Knock off aphids with a strong blast of water or some bug soap as soon as they appear.

* Pull weeds now! Don’t let them get started. Take a hoe and whack them as soon as they sprout.

* Prepare summer vegetable beds. Spade in compost and other amendments.

* Prune and fertilize spring-flowering shrubs after bloom.

* Feed camellias at the end of their bloom cycle. Pick up browned and fallen flowers to help corral blossom blight.

* Feed citrus trees, which are now in bloom and setting fruit.

To prevent sunburn and borer problems on young trees, paint the exposed portion of the trunk with diluted white latex (water-based) interior paint. Dilute the paint with an equal amount of cold water before application.

* Cut back and fertilize perennial herbs to encourage new growth.

* Seed and renovate the lawn (if you still have one). Feed cool-season grasses such as bent, blue, rye and fescue with a slow-release fertilizer. Check the irrigation system and perform maintenance. Make sure sprinkler heads are turned toward the lawn, not the sidewalk.

* In the vegetable garden, transplant lettuce and kale.

* Seed chard and beets directly into the ground.

* Plant summer bulbs, including gladiolus, tuberous begonias and callas. Also plant dahlia tubers.

* Shop for perennials. Many varieties are available in local nurseries and at plant events. They can be transplanted now while the weather remains relatively cool.

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