What to do
If you are planning summer displays in new terracotta pots, soak the pots overnight in water before planting them. Dry terracotta will, like blotting paper, absorb water from the compost and may not leave enough for newly introduced plants.
Pond plants, such as marsh marigolds and water lilies, can be split and replanted now. If you are planning a new pond, avoid extremely vigorous varieties of water lily. 'James Brydon' is well behaved and has flowers of deep rosy pink and handsome foliage, maroon when young.
]Seed of biennials such as sweet William, Canterbury bell, and wallflower can be sown now to flower next spring and summer. Seed of perennials, such as aquilegia and sweet rocket, can also be sown ready for next year. I have been sowing seed of a wildly flamboyant poppy, Papaver somniferum 'Blackcurrant Fizz' (Thompson & Morgan £1.99). They grow 90-100cm tall with huge double flowers, a mass of feathery, cut petals, deep burgundy verging almost on black. This is a biennial best sown where it is to flower. If the ground where you eventually want them to grow is unsuitable for broadcasting seed, try sowing it instead in light soil. When the seedlings are through, you can trowel up clumps to transplant with plenty of soil round them.
Pinch out sideshoots of greenhouse tomatoes and feed regularly. Sow cucumbers, marrows and melons inside or in a greenhouse, setting a single seed in an 8cm pot. Cover the pots with newspaper until the seedlings emerge.
What to see
The Japanese garden in Hammersmith Park, tucked behind BBC TV Centre in west London, was originally laid out as part of a big Japanese exhibition held in 1910. Though it has become dilapidated over the years, the garden still shows traces of the original rockwork, made with stone imported from Japan. The Tokyo-based designer, Yoshi Uchida, is helping a British landscaping firm to recreate two of the original cascades, with funding from the Japan Society. On 23 May there'll be a party in the park with Japanese food and traditional dances, to celebrate the garden's rebirth. For more information, go to japansociety.org.uk