Sowing and planting
Sow sweet peas in a cold frame or the greenhouse for early summer blooms next year.
Sow other hardy annuals (e.g. Consolida, Calendula, Centaurea, Limnanthes and poppies) in situ.
If you sowed any spring-flowering biennials such as Viola, Digitalis (foxglove) or Erysimum (wallflowers), earlier in the summer, they will now need planting out.
This is a good time of year to plant new perennials, especially towards the end of September, as the soil is still warm, but moisture levels are increasing.
Cutting back, pruning and dividing
Don't neglect hanging basket maintenance - a little deadheading, watering and feeding can keep them going until mid-autumn. Once they are past their best, re-plant as winter/spring hanging baskets with spring-flowering bulbs, winter heathers, trailing ivies and spring-flowering plants as above.
Continue to deadhead plants such as dahlias, delphiniums, roses and penstemons to prolong the display and give colour well into the month.
Continue cutting back perennials that are fading and dying down.
Now is a good time to divide any overgrown or tired looking clumps of alpines and herbaceous perennials such as crocosmias. This will invigorate them, and improve flowering and overall shape, for next year.
Take cuttings of tender perennials, such as Pelargonium and Osteospermum. These plants often do better grown from new cuttings each year. If you do not have a greenhouse, then use a light windowsill to grow them on.
Continue collecting and storing seed from perennials still forming seed heads.
Bring inside any tender perennials, such as fuchsias, gazanias, lantanas and abutilons, before frosts cause damage.
Some tall late-flowering perennials, such as asters, may still need staking to stop them being blown over in the wind.
Most perennial weeds are vulnerable to weedkiller in early autumn. Applying a product containing glyphosate will ensure that the roots, as well as the top growth, are killed. Treasured plants must be protected with plastic sheeting.
Spring-flowering bulbs are now available in plant centres, garden centres and online.
Pest and disease watch
Inspect chrysanthemums for signs of white rust.
Distortion on Phlox could indicate the presence of phlox eelworm.
Discoloured leaves on herbaceous plants, such as Chrysanthemum, Anemone and Penstemon could be leaf and bud eelworm.
Powdery mildew can still be a problem in a dry and warm September.