Don’t be a Sucker for Suckers

Orchardists know that winter pruning cuts can stimulate sucker growth the following year. The problem with suckers, of course, is that they do not fruit, and moreover, their growth happens at the same time in the season as blossoming, fruit set, and fruit fill, sapping the tree’s energy away from those essential processes. Suckers can trap orchardists in a vicious cycle: heavy winter pruning stimulates strong sucker growth, pruning those suckers heavily the next winter results in even more sucker growth, and so on.

In his interview with John Kempf on the Regenerative Agriculture Podcast, Dr. Ted DeJong noted that understanding the way a tree is designed is critical for a fruitful pruning regimen. Unlike annual plants, whose function is to reproduce and set seed as quickly as possible, trees are designed first to fill space, and only then to set fruit.

Therefore, fruit trees—in response to limb damage caused by an ice storm, animal browsing, or pruning—re-fill that empty space as quickly as possible. They do this by sending out epicormic shoots, better known as suckers or water sprouts, which grow rapidly from damaged tissue. Epicormic shoots are vegetative—their function is strictly to fill space—so they can diminish an orchard’s productivity.

Dr. DeJong says there are 2 ways to subdue suckers, both derived from a first-principles understanding of how trees allocate their energy.

1) Avoid heading cuts when possible, and use thinning cuts instead.

Thinning cuts remove stems at their base, where they connect to a branch, while heading cuts remove the tip region of a branch. Heading cuts elicit more regrowth for two reasons: they remove the auxin-producing region which causes apical dominance in the branch, and also disturb the ratio between branch biomass and buds (potential growing points).

There are more buds further out on the branch, where its radius is smaller, while there are fewer buds towards the base of the branch, where it has a thicker radius. If the branch’s radius is a rough corollary for its energy, then removing all those buds out far out on the branch means the branch is left with a lot of energy for very few buds. That energy essentially shoots out of the buds as water sprouts the following spring.

Thinning cuts remove fewer buds and thin-diameter wood, so this response isn’t as strong.

2) If heading cuts are unavoidable, then pull suckers out in spring

It helps to understand how trees store their carbohydrates at varying parts of the season. In the winter, the bulk of a tree’s energy is stored in its roots and thick structural branches. Pruning out smaller branches in the winter will remove a lot of buds, but relatively little energy from the tree. That means that in the spring, as the tree moves energy back out into its branches and buds, a lot of energy will be directed towards relatively few buds, resulting in strong vegetative growth.

By contrast, in the summer, a tree has spent much of its stored carbohydrates producing new growth, so has relatively little energy to put out through its buds in the form of vegetative growth if branches are pruned at this time.

Dr. DeJong recommends manually pulling out suckers, rather than pruning them, and doing this as early in the spring as you can, when they’re only 12 to 16 inches long. This way the tree will put the least amount of energy into vegetative growth, and focus on it fruit.

He also says that “it generally does not harm a temperate deciduous fruit tree to be pruned during almost any period of the year unless stem-infecting diseases are prevalent in a region.” If those diseases are present, then prune “during drier periods of the year when pruning wounds can dry out more quickly and resist infection.”

With these techniques in hand, growers can begin to turn the tide on suckers, and make sure their orchard’s energy is being directed towards producing abundant, high-quality fruit.

For more information about this topic, check out the following resources:

• If you have 1 hour: Dr. Ted DeJong on the Regenerative Agriculture Podcast.
• If you have 12 hours: T. M. DeJong. Concepts for Understanding Fruit Trees. CABI (2022). (Featured on John Kempf’s recommended reading list)
• If you have 3 months: Principles of Modern Fruit Science. Sansavini et al. (2019). (Dr. DeJong’s recommended pomology textbook).