With summer slowly coming to an end, gardeners will start thinking about pruning. Acer trees, however, need to be pruned with caution – only in specific months of the year.
As one of the best trees for pots, Acers are a popular addition to gardens as they bring not only an interesting shape of leaf to your space but wonderful deep colours too. Even after summer is over, these are plants that will introduce autumn colour to your garden and help keep your space looking lively.
They do require pruning, like most trees; however, it is crucial you get the timing right when doing this. Pruning at the wrong time could damage the plant and lead to sap bleeding. Not to worry, we have advice from the experts to ensure you get it right.
When is the best time to prune Acer trees in the UK?
If you do have an Acer as a patio container plant or in the ground, it will eventually need pruning to encourage proper growth.
"When deciding to prune an Acer tree, it is important to be aware that they have high sap pressure, which means that when you prune them, they will ‘bleed’ a lot of sap from where they have been cut," says Lucie Bradley, gardening expert at Easy Garden Irrigation. "The more sap they leak from a fresh cut, the more energy and nutrients are lost, affecting the health of the plant."
"At the same time, the heavy leaking of sap makes it harder for the fresh cut to heal, which makes your acer vulnerable to disease, as sweet, sticky sap will attract bacteria and fungus," she adds.
"This means that you need to ensure that any pruning that you carry out is kept to a minimum and that you prune only when the tree is in a dormant phase, as this is when its sap pressure will be at its lowest and fresh cuts will bleed the least," continues Lucie.
Because of this, Acers benefit from being pruned at the very latest in February; however, earlier is better depending on the climate.
"So you should confine pruning until your acer is in a fully dormant state, which is from November until January, so you avoid heavy ‘bleeding’ of sap. During this period, your acer will not have any leaves, which will also make it easier for you to efficiently prune as you can see the full structure of the tree," explains Lucie.
"Your only exception to pruning during this period is if you have any long, straggly stems growing and disrupting the shape of your acer. You can carefully remove these in July-August, but this should only be for odd, unruly stems and not an overall trim as the cuts would weep sap excessively," she says.
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How do I prune my Acer to shape?
If you've spotted your Acer looking a little dishevelled while sorting the garden out, luckily, you can shape it to make it look tidier.
"To prune an Acer to shape, you should firstly remove any dead, damaged and diseased wood. Then, reduce congestion in the tree's canopy by cutting out branches that cross, rub together or grow straight down towards the centre," says Richard Barker, horticulture expert and Commercial Director at LBS Horticulture.
"Work from the top down when pruning, trimming back long, awkward outer branches to just above an outward-facing bud or lower branch junction. You should not try to shear an acer like a hedge, and only remove what is necessary when pruning," he adds.
Whether you're shaping or pruning, always make sure your essential garden tools are clean and as sharp as possible. This will prevent the spread of disease and cause less stress to the tree.
Can you cut an Acer right back?
Not only should you avoid pruning in summer; it's also best not to cut acers right back.
"I would never recommend cutting an acer right back – with acers it’s all about keeping pruning to a minimum as these slow-growing trees will naturally create a graceful form," says Lucie. "If you are too harsh with pruning, the loss of sap will weaken the acer, and you will get unsightly dieback to parts of the tree which won’t recover; at worst you could accidentally kill your acer."
When should you not prune acers?
Now you know not to add pruning acers to your summer gardening to-do list; when else should you avoid taking secateurs to the tree?
"You should not prune acers during spring or early summer, as the sap in the tree will be actively flowing. This can mean that pruning cuts bleed sap heavily, which stresses the tree," Richard explains.
"You should also avoid pruning during freezing temperatures, as frost can damage pruning cuts and exposed plant tissue may freeze. Younger trees should not be pruned either in their first year, as the tree needs time to settle."
Looking to add more Acers to your garden? You can try an 'air layering' propagation method that will help you grow more without needing to spend a penny at the garden centre.
