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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Garden Checklist for week of Nov. 3

November still offers good weather for fall planting:

* If you haven't already, it's time to clean up the remains of summer. Pull faded annuals and vegetables. Prune dead or broken branches from trees.

* Now is the best time to plant most trees and shrubs. This gives them plenty of time for root development before spring growth. They also benefit from fall and winter rains.

* Set out cool-weather annuals such as pansies and snapdragons.

* Lettuce, cabbage and broccoli also can be planted now.

* Plant garlic and onions.

* Keep planting bulbs to spread out your spring bloom. Some possible suggestions: daffodils, crocuses, hyacinths, tulips, anemones and scillas.

* This is also a good time to seed wildflowers and plant such spring bloomers as sweet pea, sweet alyssum and bachelor buttons.

* Rake and compost leaves, but dispose of any diseased plant material. For example, if peach and nectarine trees showed signs of leaf curl this year, clean up under trees and dispose of those leaves instead of composting.

* Save dry stalks and seedpods from poppies and coneflowers for fall bouquets and holiday decorating.

* For holiday blooms indoors, plant paperwhite narcissus bulbs now. Fill a shallow bowl or dish with 2 inches of rocks or pebbles. Place bulbs in the dish with the root end nestled in the rocks. Add water until it just touches the bottom of the bulbs. Place the dish in a sunny window. Add water as needed.

* Give your azaleas, gardenias and camellias a boost with chelated iron.

* For larger blooms, pinch off some camellia buds.

* Prune non-flowering trees and shrubs while dormant.

* To help prevent leaf curl, apply a copper fungicide spray to peach and nectarine trees after they lose their leaves this month. Leaf curl, which shows up in the spring, is caused by a fungus that winters as spores on the limbs and around the tree in fallen leaves. Sprays are most effective now.

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