Gardening: The green party

London's secret gardens are intriguing - green and leafy squares locked up behind railings, to which few have access. Yet next Sunday, 7 June, many of these gardens will open their gates to admit the general public as part of the first London Garden Square Day. For the modest cost of a pounds 3 passport ticket (pounds l for children), we shall all be able to explore more than 40 urban retreats.

Visitors will have access to such spaces as the Dwarf Orchard Wildlife Garden, sandwiched between the inner and outer walls of Greenwich Park. A narrow strip of about an acre, it originally contained an orchard of dwarf apple trees planted by Charles II's gardener in 1662. A mulberry tree said to be 400 years old dominates the garden which is now a remarkable urban wildlife sanctuary with ponds, a tree nursery, beehives and resident foxes.

Dove Gardens in London SW5 is another small and very private place. Among the evergreens which have been planted in keeping with the original Victorian plan, refreshments will be served and an art exhibition staged.

Belgrave Square, London SW1, was laid out by the Victorian builder Thomas Cubitt and was from the outset a most desirable address. Spoil from St Katherine's Dock in Wapping was used to raise the levels of the four-acre site and create a green space. Now there is a sunken garden and pergolas covered with roses, wisteria and passion flowers. On 7 June, live music from the Zephyrus Wind Ensemble will entertain visitors as they stroll beneath the enormous plane trees.

Plane trees have played a central role in the greening of London's streets and squares. It was the Victorian horticulturist J C Loudon who wrote in Observations on Laying out the Public Squares of London that the ability of this tree to shed its bark would enable it to survive the polluted atmosphere of the city. Several such trees are to be found in Manchester Square, London W1, where visitors will be able to enjoy Pimms and strawberries while listening to the Saturn Wind Trio.

The London Garden Square Day is in itself is a triumph of enthusiasm over adversity. Ten years ago, its creator Caroline Aldiss was struck down with ME. "It was a nightmare, I felt claustrophobic in illness," she said. Her solace was to go and sit in the gardens of the square where she lived. "While I was there I thought, how lovely these gardens are - so secret, yet inviting."

Caroline knew it would be unacceptable for all the squares to admit the general public, but the idea of a special day when they could open for charity took root in her mind. After some research, she discussed the matter with Andy Wimble, Parks and Gardens advisor to English Heritage, who agreed to co-sponsor the event, as did the London Historic Parks and Gardens Trust. Endorsements began to flood in. Prince Charles, Tony Blair and Alan Clark (whose Kensington and Chelsea constituency has a large number of private gardens) were among those who sent letters of encouragement. Most importantly, more and more squares answered the invitation to join in. Caroline had a clear vision of London en fete and, above all, she wanted the event to be a celebration of local communities. Money raised from the day will go to a number of charities, meanwhile the public will get a rare opportunity to see some of the capital's hidden treasures.

For full details of participating gardens contact the London Tourist Board on 0839 123410 (49p per minute at all times). Tickets can be purchased from the garden squares on the day

weekend work

All the vegetables that hate frost can be sown now. There is at least a fighting chance that we won't have snow in June. Sow French beans, setting them three inches apart in rows 18 inches apart. Sow sweetcorn in blocks, setting the seeds 18 inches apart in rows 18 inches apart. Some growers find it an advantage to spread a sheet of black polythene over the ground first and plant through holes cut in the sheet. I hate the industrial air that black polythene brings to a garden so cannot vouch for the technique myself.

Sow biennials such as sweet william, forget-me-not, foxglove and honesty and perennials (aquilegias, violas, primulas) outside now to flower next year. Move the plants to their permanent positions in early August.

Watch out for blackfly homing in on the broad beans and pinch out the tips of the plants where necessary. Aphids are also clustering under the new leaves of my gooseberry bushes, giving all the new shoots a wrinkled, diseased appearance. I have been waiting for birds and ladybirds to deal with the problem but they are evidently busy elsewhere. I shall have to spray with Rapid (ICI) instead.

Oceans of bedding plants are now flooding into garden centres. Before you buy, check that the compost in the trays has not shrunk away from the sides and that the plants are bushy, compact, firm and a good colour. There should not be a mat of roots hanging out of the bottom of the tray. Resist the temptation to buy plants that are already in flower. They will not give such a long display as those that are allowed to settle before they start performing. Before planting, add a handful of general purpose fertiliser such as Growmore to each square yard of ground.