Mastering the 1/3 Pruning Rule: Protect Your Shrubs and Trees

The 1/3 pruning rule serves as a helpful principle for trimming established shrubs and small trees. Essentially, it suggests that you should not remove more than one-third of healthy growth in a single session, whether that's taking out one of three older stems or shortening each branch by one-third.

When done correctly, pruning offers numerous advantages: it maintains the desired shapes of your trees and shrubs, promotes new growth, and allows light and air to circulate freely. However, it's easy to go overboard with pruning tools in hand.

Sticking to the one-third guideline prevents excessive removal. But why is this crucial, and does it apply universally? I consulted gardening experts to get their thoughts.

Consequences of Cutting More Than 1/3

'Removing over one-third of a plant can lead to unpredictable outcomes,' states Jennifer Green, a botanist and gardening writer. 'The plant may experience shock, slow its growth, and could struggle to recover, if it even manages to do so. In some cases, it may produce excessive lateral shoots, creating a messy look that requires further trimming.'

'Excessive pruning weakens trees and shrubs over time, which is why it's generally ill-advised.'

Ned Cromack, an arborist from the UK, concurs. 'Frequent minor pruning is preferable to removing large sections at once,' he explains, emphasizing that 1/3 should be the maximum removed at any one time. 'Pruning creates wounds that can attract pests and diseases, potentially leading to a tree's demise. It also reduces leaf area, limiting the tree's energy capture.'

Ned adds that over-pruning can trigger epicormic growth, characterized by weak shoots that arise from stress and are prone to breakage.

If you need to significantly reduce a shrub or tree's size, consider a long-term approach, trimming no more than a third at a time to allow for recovery. 'In some cases, rejuvenation pruning may require removing more than one-third, but only if you know the specific shrub can handle it,' advises Drew Swainston.

The 1/3 pruning rule supports plant health by allowing gradual adjustments, according to Lina Cowley. This method also helps maintain the aesthetic structure of your trees and shrubs in your landscape.

Applicability of the 1/3 Rule to Ornamental Trees and Shrubs

'The 1/3 rule is a general guideline, but it may not suit every species of tree and shrub,' remarks Stacie Krljanovic, a Head Groundkeeper. 'Some varieties handle heavy pruning better than others, which require more careful, gradual trimming. It's wise to research the specific species and its growth characteristics.'

Ned illustrates that certain trees, like laurels and specific cypresses, can endure more aggressive pruning. Conversely, birches and walnuts necessitate a gentler approach, as they don't recover as easily.

Timing is also crucial, with some trees benefiting from winter pruning while others are better suited for summer. Consulting an arborist is often the safest option for both you and your tree.

Using the right tools is vital, whether that means bypass pruners, anvil types, loppers, or a pruning saw.

1/3 Rule for Fruit Trees

The 1/3 rule is commonly applicable for pruning fruit trees and shrubs, as noted by Drew Swainston. Stick to a maximum of one-third for small fruit trees and specific soft fruits. This includes blackcurrants, where you should remove up to a third of the oldest stems each year, as well as red and white currants and gooseberries, which require shortening branches by a quarter to a third while spur pruning side shoots.