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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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Garden Checklist for week of April 14

It's still not warm enough to transplant tomatoes directly in the ground, but we’re getting there.

* April is the last chance to plant citrus trees such as dwarf orange, lemon and kumquat. These trees also look good in landscaping and provide fresh fruit in winter.

* Smell orange blossoms? Feed citrus trees with a low dose of balanced fertilizer (such as 10-10-10) during bloom to help set fruit. Keep an eye out for ants.

* Apply slow-release fertilizer to the lawn.

* Thoroughly clean debris from the bottom of outdoor ponds or fountains.

* Spring brings a flush of rapid growth, and that means your garden needs nutrients. Fertilize shrubs and trees with a slow-release fertilizer. Or mulch with a 1-inch layer of compost.

* Azaleas and camellias looking a little yellow? If leaves are turning yellow between the veins, give them a boost with chelated iron.

* Trim dead flowers but not leaves from spring-flowering bulbs such as daffodils and tulips. Those leaves gather energy to create next year's flowers. Also, give the bulbs a fertilizer boost after bloom.

* Pinch chrysanthemums back to 12 inches for fall flowers. Cut old stems to the ground.

* Mulch around plants to conserve moisture and control weeds.

* From seed, plant beans, beets, cantaloupes, carrots, corn, cucumbers, melons, radishes and squash.

* Plant onion sets.

* In the flower garden, plant seeds for asters, cosmos, celosia, marigolds, salvia, sunflowers and zinnias.

* Transplant petunias, zinnias, geraniums and other summer bloomers.

* Plant perennials and dahlia tubers for summer bloom.

* Mid to late April is about the last chance to plant summer bulbs, such as gladiolus and tuberous begonias.

* Transplant lettuce seedlings. Choose varieties that mature quickly such as loose leaf.

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