Pruning six key plants in September helps prepare gardens for winter and encourages a flower-packed display next year, according to gardening experts. September is a critical month in the gardening calendar, as the soil remains warm enough for plants to establish strong roots without overheating, making it the ideal time to prepare them for cooler months ahead.
Pruning helps plants get ready for winter by removing damaged, dead or diseased stems. It also conserves plant energy, directing nutrients to the roots and core structure before cold weather arrives. In some cases, pruning can encourage more flowers or a better harvest, depending on the plant.
Wisteria and Rosemary
Wisteria needs pruning twice a year: once during winter before it flowers, and again in late summer after blooming has finished. Some experts suggest pruning in July or August, but recent dry spells across parts of the UK have made this harder. Pay particular attention to varieties such as Macrobotrys and Alba, removing wispy growth and long summer shoots to keep the plant tidy.
In a video for the Royal Horticultural Society (RHS), horticulturist Lucie Ponsford said wisteria needs regular pruning "to keep it in check and ensure good flowering." She explained: "Unlike other woody perennials, you need to prune it twice a year, once at the end of summer and then in winter, and that ensures that the wood will ripen and you'll get a great floral show come May."
On trimming wisteria in late summer, she added: "By cutting this back, I have allowed airflow through the plant and sunlight to get in and ripen up this wood, therefore allowing the buds here to develop and produce more flowers."
Rosemary benefits from pruning in September, but it is important not to cut back too hard. Never remove more than a third of the plant, as it may not recover. The aim is simply to tidy up, reshape and deadhead. Allow six to eight weeks for any new growth to harden off before the first frost.
Lavender and Raspberries
Pruning lavender in September helps stop it from becoming woody and straggly. Gardening expert Monty Don wrote on his blog that the "best time" to prune lavender is when the flowers begin to fade, which can happen any time between midsummer and the end of September, depending on the variety. He advises against waiting for seed heads to form or flowers to turn brown, as the plant needs as much time as possible to regrow before winter.
He added: "Cut back hard to a good compact shape, but be sure to leave some new shoots on each stem - lavender will often not regrow from bare wood. These new shoots will grow fast and provide an attractive and healthy cover to protect the plant in winter and provide the basis of next year's display."
Monty also recommends cutting back summer-fruiting raspberries in September. Old canes should be cut right down to ground level, leaving only the new green canes in place. These will go on to produce next summer's crop. He suggests keeping the six strongest canes and removing any that are weak or badly positioned. The remaining canes will then need to be tied in to a support structure for the year ahead.
He added: "I tie the canes with twine to parallel wires fixed strongly between robust posts, weaving round them and fanning them out them evenly as I work along the wire at each level. It is important that it is really secure, as winter winds can catch and damage them."
Roses and Jasmine
In September, use shears to trim shrub roses, English roses, albas, gallicas and hybrid perpetuals, cutting away any long or straggly stems. Monty suggests treating rose pruning much like trimming a hedge, cutting them into a neat, even shape. He explained: "Leave a compact, slightly domed bush that is about two-thirds of its former size."
He added: "In March, when you can see clearly without any foliage, you can inspect the shrub to remove any damaged or rubbing stems, but a simple shear in September is enough to keep it healthy and packed with flowers next year."
Summer-flowering jasmine should be pruned shortly after it finishes blooming, making September the right time to tackle it. Next year's flowers are likely to grow on this year's wood, so only cut the new growth back to the tip. Trim any untidy stems by roughly a third, cutting just above a leaf joint, and do not remove more than a third of the stem at any one time.
