Sacramento Digs Gardening: 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.

GENERAL GARDENING

An article about gardening.

Keywords:

Sacramento seasons light sensitivity greenhouses rudbeckia gardening solstice daylight plants bloom darkness summer dahlias flowers

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Food in My Back Yard Series

April 22: Should you stock up on fertilizer? (Yes!)

April 15: Grow culinary herbs in containers

April 8: When to plant summer vegetables

April 1: Don't be fooled by these garden myths

March 25: Fertilizer tips: How to 'feed' your vegetables for healthy growth

March 18: Time to give vegetable seedlings some more space

March 11: Ways to win the fight against weeds

March 4: Potatoes from the garden

Feb. 25: Plant a fruit tree now -- for later

Feb. 18: How to squeeze more food into less space

Feb. 11: When to plant? Consider staggering your transplants

Feb. 4: Starting in seed starting

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Garden Checklist for week of April 27

Once the clouds clear, get to work. Spring growth is in high gear.

* Set out tomato, pepper and eggplant transplants.

* From seed, plant beans, beets, cantaloupes, carrots, corn, cucumbers, melons, pumpkins, 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. Late April is about the last chance to plant summer bulbs, such as gladiolus and tuberous begonias.

* Transplant lettuce and cabbage seedlings.

* Weed, weed, weed! Don’t let unwanted plants go to seed.

* 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.

* 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.

* Start thinning fruit that's formed on apple and stone fruit trees -- you'll get larger fruit at harvest (and avoid limb breakage) if some is thinned now. The UC recommendation is to thin fruit when it is about 3/4 of an inch in diameter. Peaches and nectarines should be thinned to about 6 inches apart; smaller fruit such as plums and pluots can be about 4 inches apart. Apricots can be left at 3 inches apart. Apples and pears should be thinned to one fruit per cluster of flowers, 6 to 8 inches apart.

* 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.

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