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 Dahlia, Delphinium, Rosa and Penstemon 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.
Propagation
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.
General maintenance
Bring inside any tender perennials, such as Fuchsia, Gazania, Lantana and Abutilon, before frosts cause damage.
Wait for the first frosts to hit dahlias and cannas before lifting the tubers or rhizomes. In warmer regions, they may be alright left in the ground, but do cover the crowns with a protective layer of straw or bracken.
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.
Planning ahead
Buy spring-flowering bedding plants and buy or order spring-flowering bulbs.
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 dry, warm Septembers.