Dahlias are unapologetically late. They spend early summer building leaf and root, come good in August, and only truly hit their stride once the nights begin to cool. That timing works in your favour now. The softer light and cooler nights of early autumn hold the colour in the petals far better than the glare of midsummer, so the flowers come richer, cleaner and longer lasting. A plant that looked ordinary in July is a different thing in late September.

By now the plant is at full size, with a root system big enough to push out flower after flower, and it will keep going until your snips or the frost stops it. Every bloom you take tells the plant to make another, so this is the moment to cut hard and cut often. Dahlias are one of the great engines of the late cutting patch, and they will keep a jug on the kitchen table well into October. We wrote about that whole cast of plants in The Late Summer Cutting Garden.

The stage to cut at

Most flowers you cut in bud and let them open in the vase. Dahlias are the exception, and it is the single most useful thing to know about them. A dahlia will not open any further once it is cut. Whatever stage the flower has reached when it leaves the plant is the stage it stays at, so a bud cut too early simply sits there and then goes over without ever opening.

Cut when the flower is fully, or very nearly fully, open. Turn a bloom over and look at the back: the petals should be firm and the reverse fresh and tight, not soft, browning or damp. On the fully double types, wait until the centre petals have unfurled, because a dahlia with a hard green eye still buried in the middle has not finished. It takes a little practice to read, and you will cut a few too soon at first. That is fine.

The deep drink that makes them last

Dahlias are thirsty and they wilt quickly once cut, so the whole game is getting water back up the stem fast. Take a bucket of cool water out to the plants and stand each stem straight in it as you cut, rather than laying flowers in a trug in the sun. Cut in the cool of the day, first thing in the morning for choice.

Back indoors, stand the stems in deep, cool water out of the sun for several hours, or overnight, before you arrange them. This long conditioning drink turns a five-day vase life into a seven or eight-day one, and keeping leaves out of the water keeps it clean. The full method is in our guide to Cutting & Conditioning Flowers Like a Florist.

Through the flowering season a dahlia earns its keep in return for a good drink and a regular feed. The feed you want is a high-potash one, the same tomato feed you would give a greenhouse crop. Potash goes towards flowers, where a high-nitrogen feed would only push soft leafy growth at the expense of blooms. A weekly or fortnightly liquid feed through August and into September keeps the flowers coming thick and strong.

There is a point to stop, though, and it matters if you mean to keep your tubers. As the season turns, ease off the feed and let the plant begin to wind down. Stop feeding by early October, or a few weeks before you expect your first frost. A plant still being pushed into soft growth going into winter is one whose tubers have not had the chance to firm up and ripen, and those are the ones that let you down in store. Keep watering while the plant is in growth, and let the feeding taper off.

If something has been quietly lacemaking your dahlia petals overnight and there is nothing to be seen by day, the culprit is almost certainly earwigs. They feed after dark and tuck themselves into tight, dark places once it gets light, which is the weakness the old trick exploits.

Push a cane in among the dahlias and balance an upturned flowerpot on top, loosely stuffed with straw or shredded paper. The earwigs climb in to hide at dawn, and you tip them out each morning, well away from the bed. A few pots through a patch will take the pressure off the worst of it.

Earwigs are not the only night feeder. Slugs will shred soft dahlia growth and rasp holes in the leaves and petals, and they do their worst in the damp that autumn provides in plenty. Beer traps and a torch after dark both help, and for a control that works out of sight, in the soil where the slugs actually live, a watering of slug nematodes does the job. They are microscopic, harmless to pets, wildlife and the plants, and they see off the slugs below ground for around six weeks. Water them on while the soil is still warm.

Natural Slug Control

Slug Control Nematodes (40 sqm)

Microscopic nematodes watered onto the soil that hunt down slugs below ground, where pellets and traps never reach. Safe around pets, wildlife and children, and working for about six weeks.

£16.00 View →

Cutting flowers for the house is the best deadheading there is, because every stem you take for the vase is one the plant no longer spends on setting seed. When you are not cutting, deadheading does the same job of keeping the plant in flower. The thing that trips people up is telling an unopened bud from a spent flower that has dropped its petals, because on a dahlia the two can look oddly alike.

The tell is in the shape and the feel. A bud that has yet to open is round, firm and domed, like a small green button. A head that has been and gone is more pointed and cone-shaped, the base behind it swelling as it starts to think about seed, and it feels softer and looser. Take the spent ones off, cutting back to the next pair of leaves or a side bud, and leave the fat round buds alone.

The first hard frost is abrupt. One cold, clear night will blacken every leaf on the plant, and the top growth collapses within a day. It looks like death, and for the season above ground it is. The flowers are gone and the stems turn soft and dark.

What the frost usually does not do is kill the tuber. The soil holds enough warmth to protect the crown through a first frost, so what has died is the top of the plant, not the plant itself. The blackened growth is your signal to cut the plant down and make a decision about the winter, rather than a sign that all is lost.

There are two honest ways to take a dahlia through a British winter, and which one suits you comes down to your soil, your weather, and how attached you are to a particular plant.

Lifting and storing

Cut the blackened stems down to a few inches, lift the tuber with a fork, and shake off the soil. Let it dry for a week somewhere frost-free, then pack it into a box of barely damp compost or wood shavings with the crown left clear, and keep it cool and frost-free. Look it over through winter and take out anything that has gone soft.

Leaving in under a mulch

The easier route, on lighter, free-draining soil, is to cut the plant down and cover the crown with a thick, dry mulch of straw, bark or leaf mould to insulate it. The risk is wet rather than cold: a tuber sitting in sodden ground all winter, especially on clay, may rot before spring.

Whichever route you take, the tuber is worth protecting from below. Vine weevil grubs, fat white commas with brown heads, feed on roots and tubers through autumn and winter, and can hollow out a stored or overwintering dahlia with no sign above ground until it is too late. Early autumn is the moment to deal with them, while the grubs are small and the soil is still warm enough to treat. A watering of vine weevil nematodes onto the soil or into pots now kills the grubs before they reach the tuber, and the kraussei type we stock keeps working into the cooler weeks when other nematodes give up.

Autumn Tuber Protection

Vine Weevil Control Nematodes

Steinernema kraussei, the cold-tolerant nematode for autumn use. Watered onto soil or into pots now, it hunts down the vine weevil grubs that eat dahlia roots and tubers over winter. Safe around pets and wildlife.

From £13.20 View →

Here is the part we enjoy most. You can save your own dahlia seed, for nothing but a little patience, with one rule to understand first. Seed saved from a dahlia will not come true. Not one seedling will be a copy of its parent, because dahlias carry eight sets of chromosomes and throw their genes around like confetti. Sow a pinch of your own saved seed and you get a bed of complete one-offs: your own unnamed dahlias, no two the same, and none quite like the flower the seed came from.

For most of us that is the whole appeal, and the cheapest way there is to raise something genuinely your own. The open-centred dahlias are the ones to do it with: singles, collerettes and looser semi-doubles like Bishop's Children hold their centres open, so the bees can reach in and pollinate them and they set good seed. The tightly packed doubles hide their middles away and rarely give you much worth sowing.

To save it, leave a few good flowers on the plant to go over instead of cutting them, and let the heads dry and turn fluffy and grey. Pick them before a long wet spell rots them, finish drying them off indoors on a windowsill, and pull out the flat, dark, almond-shaped seeds. Store them somewhere cool and dry over winter, and sow them in spring exactly as we set out in How to Grow Dahlias from Seed. Next autumn, you find out what you have made.

The One to Save From

Dahlia 'Bishop's Children'

Open single and semi-double flowers over near-black bronze foliage, in fiery scarlet, orange and pink. Bee-friendly, easy from seed, and the open centres set seed readily for saving.

£2.30 View →

We grow several dahlias from seed here, and they are all a fraction of the price of tubers. Alongside Bishop's Children, we stock these:

Dahlia 'Cactus Mixed'

£2.30View →

Dahlia 'Early Bird Mix'

£2.30View →

Dahlia 'Pompone Double Mixed'

£2.30View →