What needs to be done in the garden in September depends so much upon the weather. It is one of those change-over months which can mean a continuation of summer or the onset of autumn and many tasks will be governed by how damp the soil is - i.e. rainfall - and temperature. One thing that is a constant and cannot be changed by our interaction with the environment, however, is day length. Plants (and birds and animals) respond to increasing and decreasing day length regardless of the humidity or temperature levels; the challenge for nature each year is striking a balance between these competing demands..
Bearing all of this in mind, it is obvious that for some plants flowering, fruiting and closing down at the end of the year is happening early in order to cope with the heat and lack of available moisture. Whereas for other species - such as fungi - their fruiting bodies will be waiting for rainfall before they appear.
This is what we anticipate doing this September, which is forecast to continue hotter than average and dry.
In the fruit and vegetable garden:
Continue picking, preserving and storing tomatoes and chillis, courgettes and squashes, aubergines and peppers, beans, potatoes, root vegetables such as beetroot, carrots and radishes plus apples, quince and walnuts.
Walnuts drying in the autumn sun - you must do this before you store them or they may rot inside the shell.
As vegetables finish cropping remove the green or browned off plant matter and add it to the compost heap. Dig out the roots too, unless they happen to be those of legumes (beans, peas etc). These roots host a bacterium called rhizobium which helps to fix nitrogen in the soil. Chop the roots of legumes up with your spade and incorporate them and the nitrogen fixing bacteria into the soil. Nitrogen is important for leaf growth, so if you rotate your vegetable planting it is sensible to plant leafy green vegetables where you last planted peas or beans.
Once your soil is clear of this year's plants and any weeds, and providing you are leaving it fallow over the winter, you can start to condition it for future use by covering it in home made compost, well rotted manure or chopped straw (especially if it has a bit of chicken manure in it). However, do not do this until the soil is wet, otherwise you will be obstructing the penetration of rainwater. Add your material after rain and the moisture is trapped below it. Over the winter worms will drag the organic matter down into the soil helping to condition it and making it more water retentive in future years. You can also sow a green manure which you dig into the soil next spring. All of these methods have the added advantage of suppressing weed growth.
In the ornamental garden:
Continue to dead head flowering plants such as roses, dahlias, morning glory, cosmos etc and, unless you want to keep the seed heads, cut back spent flowers and foliage and add them to the compost heap.
We leave grasses such as this Miscanthis sinensis Malepartus as it looks beautiful all winter and the seeds are welcome food for the birds. It is cut back in the spring just before the new growth appears.
Remove weeds as you work around your borders so that they do not establish themselves over the autumn months.
Continue to water plants in pots if you are able to, but be careful about feeding them, If they cannot be watered as normal cut back or stop feeding - without water the plant cannot take full advantage of the food and it can actually be disadvantageous because it will alter the dehydrated plant's chemical balance.
Order your spring flowering bulbs. Now is the time to plant crocus, daffodils, alliums, and muscari - if you can get your spade into the ground. Bulbs need to be planted so that they have about twice of their own depth in soil above them. If the ground is impossible to work either water it to soften it or wait until after there has been rain. Don't plant tulips yet - they are best planted in November.
Once the soil is reliably damp you can divide herbaceous perennials and transplant small evergreen shrubs. I will be waiting until the end of the month and into October before I do this as it is really important that when you move them the soil is moist and easy to work. They must not be allowed to dry out. You can also do this in the spring but I find that in our climate, in SW France, plants benefit from being moved in the autumn so that the roots can really establish themselves before the heat of next summer. Don't move deciduous plants yet - that is a winter job.
Now is a good time to cut hedges. Ours are cut every September/October as there is absolutely no risk of disturbing nesting birds.