Photo Courtesy Paul Keleher
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.
Magnolia buds photo courtesy Per Ola Wilberg
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 45F, but above 32F. 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.
Daffodil photo courtesy Dawn Endico
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 50F. 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 35F to 45F 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 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.
Garlic foliage
For more detailed information on the acclimation process in woody plants, read the North Carolina State University publication “Preparing Nursery Plants for Winter”: http://www.ces.ncsu.edu/depts/hort/hil/ag454.html.
