Part 1: Light Reactions
In order for plants to flower at about the same time every year, go dormant for winter and resume growth in the spring, it is vital for a plant to know the time of year. Without this awareness, plants could get caught fully leafed out, actively growing and too tender to survive the cold. As the days get shorter in the fall, plants get the cue from the change in lighting that they need to begin winter preparations.
This awareness of the time of year results from the plants ability to track daylength or photoperiod and is known as photoperiodism. Plants are grouped roughly as long-day, short-day and day neutral based on their response to photoperiod. Photoperiod is defined as the time that plants are exposed to light during a 24 hour period. A 10 hour photoperiod has 10 hours of light and 14 hours of darkness and would naturally occur twice each year, once in the late winter in early February and again in the Fall at the beginning of November. It's interesting enough just to know that plants run on a 24 hour cycle. Many plants will go into a night cycle with the lights fully on. Think of these plants as jet lagged.
Long day plants create blossoms as the days get longer. Therefore, these plants set their flower buds as soon as the days get long enough in the spring. Most sunflowers are long day plants, so they bloom in the summer. Other common long- day plants include: Petunia, Snapdragon, Black-eyed Susan, Shasta Daisy, Purple coneflower, African marigold, and Hosta.
Here's a great demonstration of photoperiod: Plant nearly any common sunflower in the late summer after about mid-July. They will grow to a very short height and make very tiny blossoms and do it all very quickly as compared to the same seed planted at the end of May. Since the daylength in mid-late summer is correct for flower development, the small young seedlings produce flowers almost immediately, compared to the plant in the spring where weeks of vegetative growth result in a much larger plant at flowering. To complicate this scenario, there are some day- neutral sunflowers grown for the cut flower trade that are much less sensitive to day length and will look pretty much the same assuming there is enough light and warmth to grow and flower. Many bedding plants are photoperiodic, but have been bred to have a reduced response to photoperiod.
Short-day plants have pretty much the opposite reaction to day length as long-day plants. These plants set flower buds and bloom as the days shorten in late summer going into fall. Chrysanthemums (mums), Dahlia and Poinsettia are examples of short-day plants. Generally, sometime after the longest day of the year, these plants begin to set flower buds, but some can set flower buds before if the conditions are right. This makes dahlia a great choice for long-lasting summer through fall color. If you dig sweet potatoes too early in the season, there will be few if any of the swollen roots that we eat since the plants are still actively making more leaves and shoots.
The mums and poinsettias that we purchase at garden centers and greenhouses in the late summer and fall have been manipulated during production to bloom late enough for a great fall and winter display. Mum manipulation consists chiefly of pinching the bloom buds back in summer making the plants not only much thicker and more compact, but delaying flowering. Many varieties of fall mums (there are exceptions) will grow very tall and bloom in late June or early July if planted in your garden and allowed to grow naturally without pinching. In the same manner many varieties of poinsettias are also day length manipulated through the use of blackout curtains.
How do deciduous trees such as Red Maples know when to put on their annual color show? As the days get shorter in the fall, these plants begin to shut down the cellular chemistry that creates the pigment chlorophyll, the green color we see all season. As the chlorophyll breaks down, we can now see the other color pigments that have been there all along, but were masked by the overwhelming amount of green. This same cue tells the maples to create the zipper or abscission layer that eventually lets leaves fall from the trees.
So far, I've used the expression short and long day, this is an easier way to talk about this subject, but is inaccurate. Plants really tell time by how long the nights are by the reaction of the red pigment phytochrome in their cells to the dark. Day-neutral plants flower when they have grown large enough to flower.
Why do trees near street lights still change color and lose their leaves? Most street lights are of the wrong color spectrum, direct their light downward, and are too weak to affect trees. Drought and other stresses on plants can affect when they begin going dormant.
Part 2: Temperature Reactions
In order for plants to flower at about the same time every year, go dormant for winter and resume growth in the spring, it is vital for a plant to "know" the time of year. Without this awareness, plants could get caught fully leafed out, actively growing and too tender to survive winter cold. As the days get shorter in the fall, plants get their earliest cue from the change in lighting that they need to begin winter preparations (part 1 explains this process in depth). Changes in temperature are the other part of the process that plants use to mark the time of year.
Steadily dropping temperatures during the fall into early winter helps plants acclimate to the coming cold weather. Before woody plants can go dormant their tissues harden as they acclimate to the changes in conditions. In the hardening process: buds dessicate partially by dropping from 80-90% water to approximately 40% and sugars increase and soluble proteins increase in plant storage tissues.Both fertilization and pruning before plants are completely dormant will delay this process since both practices stimulate new, tender, growth, thus can cause winter damage.
Once a plant goes dormant, the buds will not respond to mid-winter warm spells until they accumulate enough chilling hours. Chilling hours are typically described as a range of hours below a specific temperature. Most researchers agree to a model that measures hours below 45°F, but above 32°F. If buds do not receive enough chilling hours during winter, the plant may experience one or more symptoms: 1) delayed leaf growth, 2) reduced fruit set, and 3) reduced fruit quality.
Some common plants in our area that provide lessons in chilling include: Saucer magnolias, Star magnolias, and raspberries. The magnolias are at the very northern edge of their hardiness zone in our area. They require very little chilling, so break dormancy quite easily in even a short winter warm spell. It is not unusual to see these magnolias in bloom during February only to the see the flower petals, brown from cold damage on the ground several days later. Raspberries, on the other hand, are very well-suited to our highly variable winter weather. They have chilling requirements of over 1,000 hours, so the buds stay dormant well into spring, thus sparing raspberries the likelihood of cold damaged flowers from a few warm days.
Many perennials require specific periods of cold in order to flower the next season. This process is known as vernalization and as in light response, varies considerably among plants. This cold treatment generally requires weeks of temperatures below 50°F. Some plants require no cold period and will simply flower when the plant has grown enough or in response to daylength. Others flower without chilling, but will flower faster with more blossoms with enough cold. Yet another group of plants require a specific period of chilling to produce any flowers. Some of this group will flower immediately after reaching the necessary cold period, while others will not flower until the days are long enough.
Examples of plants that will flower after vernalization, but are not day length dependant include: False spirea, Columbine, Coral Bells, Sea pink & Bugleweed. Once chilled, these plants will flower as soon as it is warm enough and they have enough vegetation to support flowering. Bulbs such as paperwhite narcissus, freesia, and amaryllis are native to areas where winter is warm so no chilling is required for flowering. Tulips, Daffodils, Hyacinths, Crocus, Dutch iris and Scilla all require vernalization before they can bloom. As soon as they receive enough cold, they can begin growth and bloom. These bulbs respond well to 'forced' blooming indoors by keeping them at 35°F to 45°F for 12 weeks or more, then planting them and bringing them to room temperature so they can begin growing. It will take at least 4 weeks for these 'forced' bulbs to flower.
The hardneck garlic that we commonly grow in PA is another study in vernalization. Planting garlic outdoors on or about October 15 will produce excellent quality bulbs by early to mid-July. A single garlic clove planted in the fall will produce a 6-8 (or more) clove bulb depending on type and variety in the early summer. If on the other hand, you plant the same clove in March or April, it will produce tasty greens, but will not flower or produce a typical garlic bulb. Most softneck type garlics are less sensitive to the chilling requirement as they are more typically grown in warmer areas. There are garlic varieties that can be grown in nearly every climate.
Plants begin preparations for winter largely based on the increasing length of the night (shorter days), but temperature is important to the dormancy and hardening processes. They break dormancy and resume growth in the spring based on increasing temperatures.
For more detailed information on the acclimation process in woody plants, read the North Carolina State University publication "Preparing Nursery Plants for Winter".