When is the best time to prune bushes and trees?

If you readabout pruning on the garden centre Plantasjen's website, it states:

"July,August, and September are good months for pruning most bushes and trees in thegarden. The plant is in full growth, and we can see how the plant’s shape lookswith foliage."

On thewebsite of Hobbygartnerskolen (the Hobby Gardening School), you will also learnthat the so-called JAS period (July, August, September) is a great time forpruning:

"Atthis time, the plants are in full growth, and they close their wounds quickly.This makes them less susceptible to fungi and other diseases, and it is alsoeasy to shape trees and bushes while they are still full of leaves."

Severalgardeners are familiar with this JAS theory.

However,experts do not consider this to be good advice.

“Thisknowledge has spread through word of mouth,” Anna Lund says.

She is aresearcher at the Swedish University of Agricultural Sciences (SLU) and wasinterviewed by the Swedish websiteExakt (link in Swedish).

The theoryis based on the idea that it is wise to prune when the plants contain themaximum amount of energy, according to Lund.

“But infact, there is rarely a shortage of energy in bushes and trees, so from thisperspective, it doesn’t really matter when you prune,” she says.

Lund andcolleagues have tested pruning on various species at different times of theyear.

Sweetcherry, linden, maple, and pedunculate oak have been pruned at different timesof the year.

Theresearchers have investigated whether the branches became discoloured at thecut and whether the tree was able to form new branches at the cut.

Theirresults show that all four species fare best in forming new branches when prunedin January.

As for discolouration,it turned out that linden and oak were significantly less discoloured whenpruned in January.

Sweet cherrywas least discoloured when pruned in September.

Maple was minimallyaffected by the time of pruning.

Theresearchers observe significant differences between species. However, they knowlittle about what stresses trees when they are pruned, Lund explains in theinterview.

“It isexciting that pruning in January yields good results for certain species, as ithas been said that trees in dormancy are damaged by pruning,” she says.

It is bestto think ahead and minimise the number of cuts made to the tree, she believes.

“By pruningwhen the tree is young, the cuts can be as small and few as possible,” shesays.

EndreBjotveit, an adviser at Norwegian Agricultural Advisory Service, says that it isdefinitely best to prune while the tree is dormant. That is, in winter.

Pruningduring the growing season is not suitable for regular, commercial fruit cultivation,he explains.

Anexception may be apple trees.

“If thetrees grow too vigorously, it may be wise to remove some upright shoots in thesummer to allow light to reach the fruit. This is preferably done in latesummer, a few weeks before the expected harvest,” he says.

But this isa bit more of an ‘emergency measure,’ he adds.

Bjotveit'scolleague Gaute Myren warns that there are local differences in Norway. EasternNorway is more susceptible to frost problems in winter than Western Norway, forexample.

“In January,temperatures can drop to around minus 20°C in Eastern Norway, making it toocold to prune trees. Most pruning in Eastern Norway occurs from mid-February toearly April,” he says.

Myrenbelieves that most people in Norway have learned that it is not wise to prunetrees in the summer. It used to be more common to prune in early autumn, he thinks.

“Some years,there has been frost so early that the trees have struggled the following year.Therefore, pruning during this time of the year has completely ceased incommercial fruit cultivation,” he says.

In stonefruit, especially cherries and occasionally plums, large branches sometimesneed to be removed in the summer if the tree grows too much, Bjoveit explains.

“Removingsome large branches can help the tree allocate more resources,” he says.

But ingeneral, you should leave your pruning shears alone during summer.

If youprune your fruit tree during the summer season, you remove a part of the ‘productionsystem’ that builds up sugar and colour in the fruit, he says.

“It weakensthe tree more than if pruning it while it is dormant. The wound is also more susceptibleto diseases in the summer. There can be a risk in the winter as well. It isimportant that the weather is dry when pruning trees,” Bjoveit says.

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Translated by Alette Bjordal Gjellesvik.

Readthe Norwegian version of this article on forskning.no