Dormancy is a kind of armored sleep that plants go through. A plant, bud, or seed that is “dormant” is not visibly active. It is in a waiting state, a kind of suspension of life until conditions are right for active growth. “Actively growing” is the opposite of “dormant.”
As a gardener, it’s important to have a basic understanding of when dormancy begins and ends. Pruning and grafting tasks need to be done based on a plant’s dormant or active state. Some grafts can only be done during dormancy; some only during active growth. Much pruning is meant to break dormancy or to control unwanted breaks in dormancy.
The Quick Dirt on Dormancy
- Plants typically go dormant when temperatures approach freezing or when some critical level of drought is reached. Since these conditions are usually seasonal, a region’s plants go dormant on a yearly cycle.
- Often when a plant is dormant it looks dead, but it is alive. If you were to cut into the right part of it, you would find at least a small amount of living green tissue.
- The key thing a plant does during dormancy is enter a state in which it uses little water and sugar. It also has structures, like a hard seed coat or wood, to protect its living tissues and its energy reserves.
- Near the end of dormancy is often a good time to do pruning or grafting. This is because the plant contains a lot of stored energy, is less likely to be shocked by the sudden wounds, and will heal fastest when it returns to active growth.
Why Plants Go Dormant
What if you were rooted to one place out in the wild? Life would be fine in the rainy spring and milder days of summer. If you lived in a temperate forest, what would happen when the freezing, dry winter months came? What if you lived in a Mediterranean steppe grassland, where the cold months are not so cold, but long and unpredictable periods of dryness could kill you?
If you tried to go about your normal life, eating three square meals a day and hitting the gym after work, you wouldn’t make it. You would run out of energy, or food, or water, or your body would be destroyed by the environment. A plant that couldn’t stop growing in winter or a desert summer would face this fate.
Instead, plants have dormancy, a state where growth mostly pauses so that they can get by with a lot less water and sugar. They also shed a mantle of soft leaves and stems for something akin to a suit of armor: hardwood, scales, and other dried dead tissues much like the leathery dead skin of our own epidermis (think of a foot callous). Dormancy is how plants hang tough. Dormancy is a period where plants save energy.
Dormant Buds and Pruning
An entire plant can be said to be dormant, as is the case of deciduous plants in winter. Seeds too can be dormant: a seed is dormant until it begins to germinate and grow into a new plant.
In pruning, we talk about cutting back to a “healthy bud.” In practice, this is a dormant bud: a bud that is not growing now but could. Buds can be dormant for the reasons we’ve discussed: because environmental conditions are too cold or dry for it to be a good time for growing.
Buds can be dormant for another reason; if you look at the upper nodes of a woody plant, you will probably see buds, even in the middle summer when the rest of the plant is actively growing. These buds are dormant because higher up parts of the plant are telling them to stay dormant, using chemical signals.
When we prune off the plant above the dormant bud we interrupt these chemical signals. If it is healthy, it then “breaks dormancy” and grows. This is how we can prune without permanently damaging a plant, and why we use pruning to make plants fuller or change the direction of their growth.
Lower parts of a plant, such as the thick trunk of a tree or even the older branches, have dormant buds too, but these buds are buried within layers of wood and invisible. When a trunk is cut or there is massive damage to a branch these hidden buds can also break dormancy, causing “epicormic growth” from what seemed to be bare wood.
Knowing when to prune is important for the health of a plant. Because pruning can stimulate new growth that can be damaged in late fall or early winter freezes, wait until late winter when the plant is fully dormant. Pruning can also affect blooms. Spring and winter-blooming shrubs and trees should be pruned after their bloom cycle is done. If you prune in the winter, you'll remove buds, eliminating the spring bloom cycle. Summer and fall blooming shrubs usually bloom on new wood formed in spring (although it varies, as with hydrangeas.) Prune these plants when dormant.
Dormancy and Grafting
When grafting woody plants, the scion wood must be dormant. This is because the scion has to live on its own until the graft union forms to unite it to the rootstock. A scion is a small piece of plant that has no roots, no way to feed itself. An actively growing scion would use its small reserves of water and sugar before the union formed and would die.
Often not just the scion, but the rootstock too must be dormant. Whip-and-tongue, cleft, side-stub and other grafts must be done when the rootstock is dormant. The idea is to catch the stock at that time shortly before spring when it contains a high reserve of sugars and no growth of its own drawing off these sugars. When spring comes, the stock will send its sugars upwards in support of active growing—most importantly at the graft site, where healing and union can then rapidly occur.