Lathyrus 'Noel Sutton'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. ConsolidaCalendulaCentaurea, Limnanthes and poppies) in situ.

If you sowed any spring-flowering biennials such as ViolaDigitalis (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.

Lobelia in a hanging basketCutting 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.

Pelargonium cuttingsPropagation

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.

Symphyotrichum novae-angliae 'Helen Picton'General maintenance

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.

Planning ahead

Spring-flowering bulbs are now available in plant centres, garden centres and online.

Powdery mildew on applePest 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 ChrysanthemumAnemone and Penstemon could be leaf and bud eelworm.

Powdery mildew can still be a problem in a dry and warm September.


Removing fallen leavesMaintenance

Cover the surface of ponds with netting to stop fallen leaves from entering. Accumulated debris in the pond can encourage growth of algae and weeds, which will eventually harm the fish by reducing available oxygen levels.

Top up water levels when necessary, particularly during warmer weather, and continue to remove blanket and duckweed.

Remove dead leaves from waterlilies as the foliage dies back. Now is also a good time to divide waterlilies and other pond plants to increase stocks or control over-vigorous growth.

Overgrown marginal plants can just be cut back, if further stocks are not required. A maximum of 50 percent of the water’s surface should be taken up with planting.


Waterlilies can be prone to fungal problems such as crown rot and leaf spot, so nip any problems in the bud by dealing with them promptly. Remove affected leaves, reduce watersplash from fountains and, if necessary, repot the whole plant in fresh compost after first removing all rotten looking bits of root, stem and leaf.

You may need to thin out submerged oxygenating plants, as they can quickly build up and crowd the pond.

Greenhouse, conservatory & houseplants

Hippeastrum 'Picotee'Houseplants

Plant up containers with Hippeastrum (amaryllis) bulbs and plant prepared Hyacinths by 24 September to get flowers for Christmas.

Start to reduce watering of houseplants as light levels drop.


Damping off fungus on seedlingsIn the greenhouse

Damping down usually becomes unnecessary as the month progresses. It is best to do any watering or damping down earlier in the day, so that the greenhouse is dry by evening. Dampness during the cool nights could be a recipe for a fluffy grey mould (Botrytis) and damping off of seedlings.

Ventilate conservatories during warmer days but reduce ventilation once the cooler weather sets in. Use shading paint or blinds to help keep the area cool, but be ready to reduce shading towards the end of the month as light levels fall.

Vine weevil larvaePest and disease watch

September is an ideal time of the year to apply biological controls for use on vine weevil. Grubs will be starting to hatch, and soil and compost temperatures are now suitable for the nematodes to be effective. Target vulnerable plants, such as fuchsias, succulents and containerised plants.

When bringing plants indoors, check carefully for any pests and diseases they may have picked up outside, in particular red spider mitemealybug and scale insect. Inspect rootballs and compost for vine weevil larvae and treat where necessary.