The rule of one third is a guideline that you’ll see referenced in sources on good pruning practice, including this site. One-third is, generally, how much you should remove, at maximum, during a major pruning session.
The rule has another side to it I won’t cover here: one-third is the amount old canes you should remove during rejuvenation pruning.
1/3 as a Maximum When Thinning
Are you good at thinning? Are you able to take a shrub from a dense mass to an airy architectural wonder in one pruning session? If so, then the rule of 1/3 is for you, to stop you from going too far. The rule is: in one pruning session, don’t prune out more than 1/3 of the healthy wood. This maximum is really meant for established shrubs.
For an established large tree, the limit is 1/4 instead, and most of the time you won’t approach that. Big trees don’t need or thrive on major thinning in the way that multi-stemmed shrubs do.
One-third applies to the growing season: prune up to 1/3 once in a growing season, then don’t prune hard again that year. Follow-up pruning sessions that year should be just to tidy and remove bad growth.
Most gardeners don’t need the 1/3 rule! Are you afraid to thin? Does cutting out any healthy wood from a plant make you feel like you are hurting it? You don’t need the rule of 1/3, you need encouragement to get near one third!
Why Is There A Rule?
Plants live in a balance between the above-ground and below-ground parts. Lose too much of the top, such as by severe pruning, and the plant tries to regrow itself to come back into balance with the water and nutrients being pushed into it by the root system.
This regrowth is a great thing in moderation: it’s how plants heal, and how pruning can stimulate growth from dormant buds.
But when a plant loses too much of its leafy growth, it goes into a kind of shock or panic mode. It may try to also regrow from suckers, water sprouts,
The Rule of 1/3 Is Only for Established Plants
You only want to engage in hard pruning on established plants. By “established,” I mean a plant that has overcome transplant shock. Shock lasts through at least the first growing season after you plant. If you find the plant needs supplemental irrigation in non-drought periods, it is still in shock, which can linger—years—for plants transplanted when large. Don’t prune any living wood during the transplant shock period if you can help it.
Don’t Be Afraid to Get to 1/3
This is a pep talk for those of you who can’t imagine ever cutting out 1/3 of the wood on a plant, even a big, twiggy, overgrown forsythia, or some other massive shrub that has put your garden in a permanent state of solar eclipse.
To prune a vigorous shrub well, you need to go far beyond just removing dead, damaged, and diseased wood. There will be tons of crossing branches that all need to go, but you also should go beyond that and thin the shrub, selecting some strong, well-placed branches to be promoted at the expense of the less fit duplicating growth nearby, which you prune out.
Pruning 1/3 of a shrub’s wood is great under many common conditions. The more of these conditions apply to your shrub, the more aggressive you can probably be in a single pruning session, getting close to that 1/3 limit.
- Your shrub is a vigorous-growing deciduous species. Many old-fashioned garden shrubs fit this category, such as forsythia, mockorange, beautybush, and the popular viburnums. Anything commonly used as a hedge works too, such as barberry, privet, and Japanese holly. Don’t worry if it’s not on this list, judge for yourself! Any plant that you’ve had a few years and felt it could get “messy” or “out of control” is ready for a hard pruning.
- Your shrub is multi-stemmed and produces new stems or canes each year. Lilacs, rugosa roses, and other plants that commonly sucker are naturally prepared for hard pruning. They can replace wood from their strong root systems.
- You are pruning in late winter or early spring. When you are going to prune hard, this is the time of year that plants take it best. Of course, if your plant is spring-blooming, make sure you can remove your 1/3 and leave behind enough flower buds for a spring show. Often, it is possible to do both.
Be bold. It’s important to remove enough wood to achieve your goals. After teaching volunteer gardeners and interns for years, I find consistently that new gardener’s tendency is to prune too little, and fall short of the ideal.