7 Dahlia Tubers from Sarah Raven for a 5-Metre Late-Summer Border

Spacing seven tubers across five metres

Do the maths before you dig. Five metres divided by seven gives roughly 70cm between plants if you space them evenly, and that is fine for compact varieties like Bishop of Llandaff or the smaller pompons. It is too tight for the big dinnerplate types. A single Cafe au Lait or Labyrinth will throw a canopy close to a metre wide by late August, and at 70cm centres two of those will be fighting for light by the time they should be flowering hardest.

The usual fix is to mix sizes. Put the three largest tubers at 90cm to 1m apart toward the back or middle of the border, then slot the four smaller ones into the gaps at the front where their shorter habit will not be swamped. That breaks the even-rank look as well, which matters because seven dahlias in a straight line at identical spacing reads as a nursery bed, not a border. Sarah Raven’s collections often pair a tall focal variety like Karma Choc with shorter bedding dahlias precisely so the spacing can vary without leaving holes.

Mark the positions with canes before planting. Once the foliage is up you cannot judge the gaps, and shifting a tuber in July means cutting roots.

Getting tubers out of storage and into growth

Tubers from a March or April delivery usually arrive dry, sometimes with a little growth already showing at the crown. The eyes, the small pink or white buds, sit where the old stem met the tuber clump. If you cannot see any, sit the tuber on damp compost in a frost-free spot for a fortnight and the eyes will declare themselves. A tuber with no visible eyes and a soft, wrinkled neck is usually dead, and no amount of warmth brings it back.

Starting them in pots indoors from mid-March buys you four to six weeks of growth before the last frost. Use 2 to 3 litre pots, multipurpose compost, and bury the tuber so the crown sits just below the surface. Water once, then leave it nearly dry until shoots appear, because a wet dormant tuber rots faster than almost anything else in the potting shed. Once shoots are up, the pot dries out quickly and needs checking every couple of days.

If you skip the pot stage and plant straight outdoors, wait until the soil has warmed and frost is genuinely past. In most of the UK that means mid to late May. A tuber planted into cold wet May soil just sits there, and the slugs find the first soft shoots before you do.

The slug problem nobody warns you about enough

Emerging dahlia shoots are slug sweets. A tuber can push up three fat shoots one evening and have all three sheared to stubs by morning, and unlike a chewed leaf, a destroyed growing shoot sets the whole plant back weeks.

The pot-start method helps because you can grow the shoots to 15cm under cover, past the most vulnerable stage, before planting out. Wool pellets or a copper ring around each newly planted tuber buys a few nights. Nematodes watered into the soil in late spring are the only thing that has reliably cut losses for me across a wet season, and they need reapplying every six weeks because they do not persist.

Feeding, and why nitrogen is the wrong answer

Most people reach for a general fertiliser high in nitrogen and end up with magnificent foliage and almost no flowers. Dahlias respond to that the way a hedge does. They grow leaf. A tomato feed, or anything with a high potassium ratio, pushes flowering instead. The numbers on the bottle to look for are a low first figure and a high last figure, something like 4-3-8.

Work in well-rotted manure or two-year leaf mould when you plant, then switch to liquid potassium feed every ten to fourteen days from the moment the first flower buds form. Stop feeding in early September. There is no point pushing growth the frost is about to take.

Leaf mould earns its place here for a reason worth understanding. Fresh autumn leaves are too high in carbon and lock up nitrogen as they break down, which is why a one-season heap does nothing useful and can even rob the soil. By the second autumn the same heap has collapsed to a dark, crumbly material that holds water and opens up heavy clay without feeding it much. That low-nutrient, high-structure quality is exactly what you want around a dahlia, because it improves the root run without driving the leafy growth that nitrogen-rich manure can cause. A two-year-old heap is the useful one. A one-year-old heap is just wet leaves.

The practical version: keep two leaf bins going, one filling and one maturing, and only ever dig from the older. People who run a single bin always end up using it too soon.

Staking before the plant needs it

Get the stakes in at planting time. A dahlia at full size in a windy August is too top-heavy to stake without snapping stems, and a single gust can flatten a metre-wide plant in seconds. Drive a 1.2m to 1.5m cane or a metal grow-through support next to each tuber when you plant, while there is nothing to damage.

For the big varieties a three-cane tripod with twine woven between works better than a single stake, because the flower stems spray outward and a central cane only catches the middle. Tie loosely with soft twine, checking every fortnight, because dahlia stems thicken fast and a tight tie in June becomes a tourniquet by July.

Pinching, cutting, and the late-summer payoff

When the plant reaches about 30cm with three or four pairs of leaves, pinch out the central growing tip just above a leaf pair. It feels brutal on a plant you waited two months for. The result is a bushier plant with far more flowering stems, and for a border where you want mass colour from August into the first frost, that trade is worth it every time.

After that, the regime is simple and relentless. Deadhead constantly. A spent dahlia flower and a fat unopened bud look alike at a glance, so learn the difference: the spent one is conical and firm, the bud is round and soft. Cut spent heads back to the next bud or leaf joint. A plant left with seed heads decides its season is over and stops flowering, sometimes within a fortnight, so a missed week of deadheading shows up clearly in reduced bloom.

Cutting for the vase does the same job as deadheading and gives you the flowers indoors, which is half the point of growing them. Cut in the cool of early morning with a long stem, strip the lower leaves, and they last five to seven days in water. The more you cut, the more the plant produces, so a border of seven dahlias can supply a house from August onward without ever looking stripped.

The first frost ends it. Foliage blackens overnight. In milder areas you can leave tubers in the ground under a thick dry mulch of bracken or straw and gamble on the winter, but a wet winter rots more tubers than a cold one. Lifting them after the first frost, cutting the stems to 10cm, drying the clumps off for a week, and storing them in dry compost in a frost-free shed is the surer route, and it is how you turn seven first-year tubers into fourteen or twenty the following spring by dividing the swollen clumps.

What the catalogue photo does not tell you

The variety names sell a colour, but the same dahlia grown in two gardens 50 miles apart can flower a fortnight apart and at noticeably different heights depending on soil and exposure. Which raises a question worth holding onto across the season: when your seven tubers come up at different rates and your tallest swamps your shortest, do you stick with the spacing plan you drew in spring, or move plants while you still can and accept the check to their growth?