Advice
May's weather
The last frosts
With the tulips fading and the herbaceous border growing in leaps and bounds, it is now clear that summer is on the way. Surprisingly, though, there is often considerable variation in the weather experienced across the country this month, with the northern half of the UK being much colder than the southern half.
Weather averages in south Wales and for much of England below Birmingham are fairly similar. Here, sowing and planting out bedding can begin in the middle of May, as frosts are unlikely. However, in the rest of Wales, much of northern England and Norfolk, these tasks should be delayed until late May. Further north and in Scotland, it may be necessary to wait to mid-June or even longer.
Temperature
Daytime temperatures in southern England average 16°C (60°F), and some very warm May afternoons can reach 27°C (80°F) in mild regions.
During the night, temperatures average 6.6°C (44°F), but gardeners should still be prepared for a chance of frost, depending on the region in which they live. In Scotland, two frosts can be expected during May, and daytime temperatures of only about 13°C (55°F). By contrast, Cornwall and south Devon are extremely unlikely to experience any frost in May. Central London is often frost free owing to its shelter and stored heat, but suburbs and surrounding countryside are not protected in this way, and are at risk of frost.
May is very changeable. With hot and cold temperatures following each other in rapid succession, the risk is that warmer weather will result in soft growth, which is then susceptible to scorch when frosts and cold, drying winds return. Make sure you harden off your bedding plants carefully to ensure they don’t suffer when making the transition from greenhouse to garden.
Ventilate your conservatory or greenhouse in the daytime to prevent temperatures soaring in the sunshine. Be ready to apply shade paint, netting or use the blinds.
Rain
The middle of May can be one of the driest times of year, particularly in southern England. Curiously, though, for most parts of the UK, monthly averages are similar to April values - just over 5cm (2in) of rain in the south, and 8cm (3in) in western England and Scotland.
This is generally not enough to keep the soil thoroughly moist, so in most gardens it will begin to dry out. Initially, this is a benefit because the soil will reach an ideal consistency for seed sowing. However, as drying continues, there may be insufficient water to keep the seedlings going, especially in dry regions such as southeast England and East Anglia.
Use water butts to collect rainwater. Open rain-catchers are a start, but fixing the butt to your down-pipe will enable you to collect rainfall from your house or shed roof as well. Target your watering to when and where plants need it - often at fruit set and fruit swelling times, or coming up to flowering.
The last few years have seen generally below average rainfall across the UK, although in some years Scotland and Northern Ireland have avoided dry summer weather. One model of climate change predicts that, although rainfall is falling overall, the southern half of the UK could experience increasingly severe drought while the northern half experiences increased flooding.
Wind
Generally the winds are now much lighter, and even Atlantic depressions are fairly weak. The only blips are thunderstorms, which bring squally winds and damaging hail that can pockmark new foliage and flowers. Precious plants can be protected with fleece, but it is hard to predict when storms will arrive. It is also worth ensuring that herbaceous borders are well staked, to prevent gusts from flattening your perennials.
Light
Plenty of strong sunshine should ensure that plants grow rapidly this month.
In Scotland and Wales, May averages as the sunniest month of the year, with around 170-180 hours of sunshine. In southwest and south-central England over 200 hours can be expected, but light levels will increase further towards mid-summer. The Midlands follow a similar pattern, but with on average about five to 10 fewer hours of sunlight than southern England.