Late summer progresses to autumn
September is generally a cooler, gustier month than August. The days are noticeably shorter, especially in the north of the UK. Frosts can be expected in colder, more exposed parts of northern England and Scotland. But warm and sunny days are still frequent in the southern counties of England, especially at the beginning of the month.
Temperature
The first half of the month is usually warmer than the second half. Temperatures on the warmest days can often reach 19-20C (66-68F) in the south of England, 17-18C (63-65F) in northern England, and 14-16C (57-61F) in Scotland. But cooler days in September are only likely to reach 7-11C (45-52F), with 9C (48F) being an English, and 7.5-8C (45-46F) being a Scottish average for such days.
The growing season can often be extended well into September, but only in southern parts of the UK.
Applying a mulch in late summer will help to keep the warmth in the soil for longer into the autumn.
Occasional night frosts are common in late September, even as far south as Surrey, and particularly in exposed areas, on high ground, or in frost pockets. Have some horticultural fleece handy for the odd night when it is needed before the garden is fully prepared for winter. In exposed and northern areas, winter preparation should already be well underway.
Rain
Recent years have seen lower than average rainfall figures in September for most parts of the UK, with the exception of some areas in Scotland. However, although the total rainfall may be less, stormy and showery weather is still common, and very noticeable in impact, with flood risks and sudden peaks and troughs in water levels. Staking and protection of vulnerable plants will be necessary. Mulching and soil cultivation will help to smooth out the variations in rainfall, reducing pooling and drought in equal measures.
Northern Scotland has the most rain - anything between 100-200mm (4-8in) - while the south east of England and East Anglia have the least.
Average September rainfall in the southeast is usually around 60-75mm (2-3in), but, for the last three years, it has not actually risen above 40mm (1.5in) in September, with a low of 12mm (0.5in) in 2003. Watering is therefore still high up on the weekly gardening agenda. Use stored rainwater or recycled grey water wherever possible, and be aware that plant water use lessens as the autumn advances. You will need to gradually reduce watering in step with changes in your local weather, perhaps watering shrubs every 14 days rather than every five days and container plants once a week instead of every couple of days, as temperatures get cooler and rainfall increases.
Wind
September has a more blustery, autumnal feel than August. This is particularly so in the western isles, the West Country, and Wales. Although south-west winds generally prevail in the UK, the more turbulent weather systems that emerge later in September can result in northerly and easterly winds as well, which have a colder edge.
Windy weather after a dry summer can lead to early leaf fall, with leaf clearance becoming time consuming by the end of the month. Staking, pruning and tying-in of climbers should be priorities if windy weather does occur, and will be good preparation for the winter ahead.
Light
Shorter days, falling light levels, and the lower angle of the sun in the sky will limit growth rates, even on the warmest days. A well-positioned greenhouse can maximise the available light to good effect, and a vegetable plot with a favourable aspect will improve the possibilities for late cropping.
Average light levels at this time of year are 140-150 hours for England and Wales, and 110-115 hours for Scotland and Northern Ireland. However, regional variations are extreme, with 200 hours being possible in the south of England during a good year.
The quality of the light at this time of year is enough to encourage continuation of late flowering, but may be insufficient for new young green growth to flourish. Seed sowings and tender softwood cuttings may therefore fail, or else will not come up until next spring. Semi-ripe cuttings (taken from riper wood) are more likely to be successful than softwood cuttings, as they have greater food reserves and are more resistant to rotting, so can better withstand cold conditions.