A series of summer garden tours is on offer at Britain's oldest botanic garden - the Oxford Botanic - founded in 1621. The next tour takes place on 8 July and looks at the ways that different plants have been used by people through the ages. Tours start at 7pm and cost pounds 5. Meet under the Danby Arch at the Botanic Garden. For further information contact Louise Allen at the Oxford Botanic Garden, Rose Lane, Oxford (01865 276920).

The Festival of Gardening that takes place this weekend at Hatfield House in Hertfordshire celebrates the house's 500th anniversary. Lady Salisbury has made one of the best gardens in England at Hatfield, sympathetically restoring them to reflect the history of the house but filling them with the best of today's plants. It is a masterpiece. Thirty nurserymen put up displays in a grand marquee and there are demonstrations and lectures on gardening matters all through the weekend. The festival is open today (10am-6pm) and tomorrow (10am-5pm), admission pounds 5.20.

Roy Lancaster, no mean plant hunter himself, celebrates plant hunters of the past in a lecture to be held at The Lost Gardens of Heligan on 9 July. The evening starts at 6pm with a guided tour of the garden, relating its plants to the people who first discovered them in the wild. Supper is available from 7.15pm and the lecture begins at 8pm. Afterwards there will be a film on the life and work of Frank Kingdon Ward, who introduced the fabulous blue Himalayan poppy, Meconopsis betonicifolia into the country. The evening finishes at 10pm. Tickets, pounds 20, are available from Heligan, Pentewan, St Austell, Cornwall PL26 6EN (01726 844157).

Patrick Mead from Cranleigh writes with a query about hibiscus. In a new garden, he has inherited a hibiscus, which he thinks is the deep pink- flowered `Woodbridge'. The problem is that it has grown too tall, presently standing at about 12ft. He has already topped it a few times, but hopes to be able to keep it permanently at a more manageable 8ft. In Weekend Work, he noted that I had suggested pruning hibiscus by cutting out one third of its stems. His, he says, does not have enough stems for that kind of treatment. What should he do?

The method I suggested - taking out one stem in three - works for hibiscuses that are multi-stemmed rather than those that are trained as small trees, where you do not have the option of new wood springing from the base. But Mr Mead could start by cutting out entirely any branches that spoil the line of the tree or shrub, or that are getting in the way of other plants.

Because he wants to keep his hibiscus at a particular height, he will probably have to go on cutting it each year to keep it within bounds. I would cut out the tallest branch entirely each year and reduce the length of the others.

You can cut back main branches on established hibiscuses by at least a third and shorten laterals if necessary to two or three buds. You can also take the tougher course of cutting the whole shrub back by two thirds. The best time to prune is late spring, which with us generally means May.