Action plan: Nigel Colborn’s essential jobs for your garden this week

SOW DAHLIAS FROM THE SEED 

Dahlias have been flowering since late June. To keep the flowers coming, you’ll have been removing spent blooms.

But if you fancy raising dahlias from home-saved seed, you can. The seeds will germinate readily and could provide interesting and beautiful progeny for next summer.

To collect your own seed, leave just two or three spent flowers untouched on each plant. You can still sustain flowering by dead-heading the rest of the plant, allowing just those few seed heads to mature.

When ripe, the flower heads will have shed all petals and begun to turn from green to tan. The dark seeds will be visible, folded into the drying heads. Gather those heads into an old paper envelope and put them somewhere warm to dry.

Later, when the seed heads are dry, gently break them up and gather the dark, oblong seeds. You can sow those next spring in a seed tray or in small pots in a greenhouse. To germinate, they need a minimum night temperature of 16c.

If sown early, and planted out from late June, the seedlings will flower from late summer. They will not be exactly the same as their parents, and they could even be prettier.

For now, you could treat your flowering dahlias with a late summer boost. Sprinkling their leaves with diluted seaweed extract could be just what they need. Seaweed tonic works well as a dilute foliar feed. Follow the instructions on the bottle.

CHECK TOMATOES

Green house tomatoes, if they’ve been producing fruit since July, may be getting tired. Make sure they are regularly fed with high potassium fertiliser and keep harvesting the fruits.

Tomatoes are vulnerable to greenhouse white-fly.

I control mine by growing tagetes — tall French or African marigolds — among my tomato plants.

LAVENDER TRIM

LARGE lavender varieties will have finished flowering. To keep your bushes or lavender hedges compact and stocky, trim away all the spent flower heads, cutting down into the fresh foliage below the naked flower stems.

If you do that now, your lavenders may produce a sparse flush of late blooms. As well as looking pretty, those provide much-needed sustenance for pollinating insects.

Mature lavender flowers, if left on the plants, will produce fertile seed. You can pot them up or transfer them to a temporary nursery bed.

READER’S QUESTION 

My dark-leaved smoke bush Cotinus coggygria Royal Purple is ill. Entire branches are dying. Why?

Mr. J. Foster.

Smoke bushes are highly susceptible to verticillium wilt. A soil-born fungal disease, it attacks the shrub’s circulatory system, preventing water and nutrients from reaching some of the branches.

Sadly, verticillium wilt is incurable. The fungus remains in the soil near your bush. Your plant may last a little longer, but it will steadily lose vigour as the die-back spreads. Perhaps it’s better, therefore, to remove the poor thing now.

If you replace your Cotinus, chose a verticillium-resistant shrub. Holly is immune and comes in a vast variety range. Female varieties such as gold-variegated Madame Briot produce berries. You can keep holly pruned to any size or shape. For info on other resistant plants, see rhs.org.uk.

 PLANT OF THE WEEK

If you have lilac-pink or mauve colchicums in your garden, here’s a variety with a difference. C. byzantinum is a vigorous species which, when mature will produce big clumps of crocus-like flowers. Those, too, are pale lilac.

The variety Innocence, however, has pure white flowers whose tepals are tipped with lilac purple on their outer edges.

The tips are most noticeable when the flowers are closed, either in dull weather or early morning.

Like many colchicums, these are bulbs for deep, fertile soil that drains freely, but does not dry out too rapidly. A sunny spot is best, but partial shade is also acceptable. Try avonbulbs.co.uk.