David Lock and his team hope to be finished in October
An operation is under way to prune the famous yew hedges at Chirk Castle, and help the fight against cancer at the same time.
The annual task takes up to eight weeks to finish, and results in about three tonnes of clippings.
They are then bought by a company which transports them to be processed into chemotherapy drugs.
The castle's head gardener, David Lock, said it was "brilliant" to think they were helping develop anti-cancer drugs.
The drugs docetaxel and paclitaxel are developed from yew trees.
Both drugs can also be made synthetically, but yew needles are still collected and used across Britain.
At Chirk, they are bought and collected by Doncaster-based Friendship Estates, which claims they are used as the raw material for anti-cancer drugs, particularly breast and ovarian cancers.
With their electric trimmers, the gardeners hope for good weather
The company's website said clippings should be one year's growth, because the required chemical is concentrated in greener areas of the plant.
Chirk gardener Mr Lock, who started trimming on Monday, said: "We start in the third week of August and finish some time in October.
"The hedges are the most important thing at this time of year - we have to try and fit everything else in between."
The gardening team have people on the ground, as well as staff in a cherry picker to reach the top of the hedges.
Although they hope to be finished by October, rain could delay them.
Mr Lock said: "We use electric trimmers because they're lighter, less noisy and you're not constantly breathing in fumes.
"But it does mean that we need dry weather.
"It's brilliant to think the clippings are collected from here and then used for research into anti-cancer drugs."
The statuesque hedges, which line the main gardens and are dotted throughout 11 acres of grounds, were planted in 1872.
The castle, built by Edward I, is more than 700 years old and is surrounded by 500 acres of parkland.
Debbie Coats, clinical information manager at Cancer Research UK said: "There are two common chemotherapy drugs developed from Yew trees.
"One of them, docetaxel (Taxotere), first made from the needles of the European yew. The other, paclitaxel (Taxol) and was made from the bark of the Pacific yew.
"These are used to treat some breast, prostate and ovarian cancers. Both drugs are manufactured in the lab but the needles are collected and sold to the drug industry for this purpose."
Friendship Estates has been asked to comment.