A lovely month, when the light levels intensify and things come on in leaps and bounds.

Borders

Prune spring-flowering shrubs to prevent them from becoming overgrown, giving them time to make their flower growth for next spring.
Finish sowing hardy annuals: staggered sowing can lengthen the flowering season
Put support frames into herbaceous borders to support plants with a floppy habit i.e., Lupins and  Paeonies particularly
Start to harden off bedding plants by putting them out for the day and back in for the night. Keep an eye on the forecasts
Protect susceptible plants from slugs, bird friendly remedies include beer traps
Clear out spring bedding from beds and borders when it fades, adding it to the compost bin, and fork in fertiliser.
Hanging baskets can be made up this month, keeping them in the greenhouse until the risk of frost is gone
If it turns warm, keep an eye out for any pests and diseases on all plants.
Deadhead spring bulbs (but do not cut the foliage back yet)
Check that new plants don’t dry out even if it rains.  
Prune spring-flowering shrubs to prevent them from becoming overgrown.

Fruit and vegetables

Mulch around Strawberries to prevent the fruit being spoiled, use straw or black polythene
Remove and destroy any leaves of Peach and Nectarine which show signs of leaf curl
Spray a systemic fungicide on Gooseberries if mildew is a problem
Water newly planted fruit trees and bushes in dry spells
Harvest any early Rhubarb
Most vegetable crops can be sown this month, harden off greenhouse sown crops in a cold frame
Plant main crop potatoes
Earth up early crop potatoes, weeding any other vegetable crops that might need it.
Sow Dwarf Beans this month
Lettuce Varieties can continue to be sown.
Most Peas should be sown in this month
Harvest any Asparagus spears that are showing

Lawns

Apply a weedkiller and fertiliser combination
Seed new lawns and cover to keep birds away, re-seed damaged areas
Mow lawns once a week if needed. Avoid cutting if there is frost on the grass
It is now a suitable time to spot treat lawns if no general lawn weed killer is used
Lay new turf and keep well watered if a dry spell ensues

Greenhouse

Tie in the lengthening growths of Vines
Remove Insulating bubble if not already done so.
Ventilate well and damp down on sunny days as long as seed are not being raised
Continue pricking out half hardy and tender seedlings
In a heated greenhouse remove side shoots from tomatoes, but not bush varieties
Pinch out side shoots of melons and cucumbers when two side shoots have been formed
Watch for pests and treat immediately using either a chemical spray or a biological control
Ensure heating is still available in case of a cold night, keep some fleece handy just to pop over if a frost is forecast

Ponds

Plant up new aquatic baskets with Water Lilies and oxygenating plants
Feed fish if they are near the surface
Keep pumps and filters running constantly, clean as necessary.
If the water has warmed sufficiently, now is the ideal time to add fish.

General Tasks

Control slugs and snails by trapping
Visit the garden centre for new pots and basket plants
Watch out for late frosts and keep fleece handy
Check watering each day as temperatures rise
Look at you garden - see how much it’s changing

Plants which are at their best in May

Aquilegia (Granny's Bonnet)
Azaleas
Ceanothus vars
Choisya
Enkianthus
Syringa vulgaris cultivars (Lilac)
Ornamental Cherries
Camassia - bulbs
Crataegus (Hawthorn)
Clematis Macropetela (Clematis)
Centaurea Montana (Perennial Cornflower)
Berberis stenophylla (Barberry)
Dodecatheon Meadia (Shooting Stars)
Kerria (Sailors Buttons)