After severe frosts last winter, gardeners may be understandably twitchy as temperatures decline. Astronomical autumn started on the 22 September – the date when day and night are both 12 hours – and this corresponds to horticultural autumn too, when plant growth and gardening activities slow as night length exceeds day length.
Plants only grow outdoors for part of the year in Britain, called the growing season. This is defined scientifically as beginning when there are five consecutive days at 5°C or more, and ending when there are five consecutive days at less than 5°C, typically 280 days. This is 29 days longer than in the period 1960-1990, with most of the increase at the end of summer. This accounts for better results with late-maturing crops such as grapes, squash and sweetcorn.
The old gardener’s rule of thumb of last spring frost and first autumn frost defining the growing season is still useful though, because different sites differ in how prone they are to frost with low-lying areas more vulnerable due to cold air flowing downhill into valley bottoms or against walls and hedges running across the slope.
Grass frosts can happen in September, where ice forms on grass but not on concrete due to the latter’s ability to store heat. These can do damage to low-growing very tender plants such as begonia and cucumbers. Gardeners can deploy cloches or fleece, or bring potted plants into greenhouses or indoors to protect plants.
However, by late September the growth rate and leaf health of very tender plants is poor and it may be best to clear ground or pots and replant with winter bedding, wallflowers for example, or cover crops to improve veg plot soil while storing begonia tubers for next year.
As temperatures fall, ice forms on the ground, shed roofs and trees as heat is radiated away during the lengthening nights, with mostly low-growing vegetation at risk of damage.
Soil acts as a “heat bank” where warmth from the sun is stored in the soil to be released at night. Dry soil holds less warmth than moist soil, so it is worth keeping tender plants watered in September. Not only does moist soil store more warmth, but there is still potential for growth if plants are not short of water. This no longer applies come the cooler, duller days of October.
Unlike tender, easily chilled, spring and early-summer growth plants, even relatively tender garden plants are quite frost-tolerant now. Late sweetcorn, blight-resistant tomatoes, salvias, penstemon and dahlias, for example, keep going until air frosts occur.
Colder nights are not entirely unwelcome once plants have reached their full potential. Blight-resistant potatoes will not grow much after September, quality declines and slug damage can occur. Destruction of their foliage by a timely frost aids harvesting potatoes.
Air frosts are when the temperature falls to freezing at least one metre above ground level. Fleeting air frosts do little damage, but by mid-October nights are long enough for air frosts to be a risk, especially in northern regions, although November is more common in the south and last year there were no significant frosts until December.
Using a mixture of local experience and weather forecasts, wise gardeners will barricade vulnerable plants such as abutilon, bananas and canna that are not brought indoors with straw, hessian and fleece before severe air frosts occur.
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