Answer: When leaves turn reddish out of the autumn season, the plant is showing stress. The most likely reason is that the container dried out during summer. Check to see if next year's buds are there, all ready and waiting to burst into life, and in April, repot the acer into fresh ericaceous John Innes compost. If the plant is already in the largest container you can manage, cut away up to one fifth of the roots, and repot back into the same container. The leaf edges turning brown are again, a symptom of dry compost. During the growing season, it's a good idea to check the compost every day to feel whether it's moist, and when watering, keep going until the water comes out of the bottom of the container.
Send your questions for the RHS to: firstname.lastname@example.org.
Only a few questions per month can be answered. For further advice on handling problems in your garden, visit www.rhs.org.uk/advice.