June 23, 2017
This is the week of the summer solstice, also known as midsummer (the halfway point of summer). The solstices and equinoxes have traditionally been important to gardeners because of their effect on the flowering habits of some of the most important plants in the garden.
Plants subject to photoperiodism use the length of the dark periods (nights) as a cue to releasing hormones for flowering and fruiting.
Due to a misunderstanding of the actual triggering stimulus, the terms “short day” and “long day” were chosen to describe the two types of plants that are photo dependent for flowering. A good way to refer to them would be “long night” and “short night” plants since it is actually the length of the dark periods and not the length of the light periods that trigger flowering.
Short day/long night plants such as chrysanthemums will flower in fall or spring (and winter). Some plants like tomatoes are “day neutral” and use a different mechanism to trigger flowering (such as chronological age).
In outdoor gardens, spring, fall, and winter all have longer nights than the short nights of summer. Short day/long night plants bulk up during the growth stage of summer, until the longer nights of fall trigger flowering. If these plants are set outside too early in the spring months when the nights are long, they pick the cue from the long nights to start flowering.
The four seasons are important plants that use the length of darkness “Night” to trigger flowering.
The Summer Solstice (Midsummer)
The Summer Solstice is around June 21st and has the longest day and shortest night, artificially lit gardens often use light to dark periods of 18 on 6 off to simulate this lighting condition and prevent flowering in long night plants.
The Autumn Equinox
The Autumn Equinox is around Sept 22nd when the longer nights and shorter days again approach 12-12. This is the traditional fall harvest season, As the weather cools, keep an eye out for bud rot and other fungal diseases, which can thrive in the moist, cool fall.
The Winter Solstice
The Winter Solstice is on December 21st and marks the day of the year that has the longest night (Take note that the dates given are reversed south of the equator, where midsummer occurs in December, and Midwinter is in June). While in many areas the cold of winter prevents outdoor gardening, it is an excellent time for indoor gardening, as cooler outdoor temperatures usually make indoor lighting heat issues easier to cope with.
The Spring Equinox
The Spring Equinox is around March 20th, when day-night approaches 12-12. Depending on the date of the last frost, some areas can use photoperiodism triggered plants for an early “spring harvest”.
To replicate seasonal lighting indoors for long night plants, many gardeners use 0-6 hours of darkness to simulate the short nights of summer, encouraging short night plants to grow and develop without flowering. When the plants have developed sufficiently, the dark periods are increased to 12 hours or so, giving them “long nights” and triggering flowering.
Image courtesy of Nick Kenrick