Dahlia

Light and exposure

The dahlia demands full sun. Give it the brightest and warmest spot in the garden, with at least six hours of direct sun a day. That is the condition for abundant flowering and nicely upright stems.

In shade or in a spot that is too cool it runs up tall, flowers sparsely and flops. An open, airy position also helps limit the powdery mildew that readily settles in late in the season in a damp climate.

Watering

This is a plant with a big appetite for water throughout its growth. Water regularly and generously right through the growing season, especially in hot dry weather, aiming at the base rather than the foliage. A dahlia short of water slows its flowering and makes smaller flowers.

Be careful though at the very start, just after planting: the freshly set tuber rots easily if it sits in cold damp. Water sparingly until the first shoots appear, then move to a more generous regime once the plant really gets going.

Soil and repotting

The dahlia wants a rich, deep soil, well worked and above all well drained. Enrich the planting area with mature compost or well rotted manure, it has large needs to sustain flowering that lasts for months.

Drainage is decisive, especially for tubers left in the ground where the climate allows it. On heavy clay soil, lighten with compost and coarse sand, or plant on a small mound to keep excess moisture away from the crown.

Temperature and humidity

The critical point. The dahlia is a plant of the Mexican highlands, it likes warmth and tolerates no frost. A tuber left outside is destroyed as soon as the soil freezes, and the foliage blackens at the first frost.

This frames the calendar on both sides: you plant only two to three weeks after the last frosts, once the soil reaches 15 degrees, and you lift as soon as the first frost has blackened the foliage. In between, it makes the most of a mild summer, provided it has sun and water.

Pruning and upkeep

Two actions extend and thicken the flowering. Pinch the main stem above the third or fourth pair of leaves when the young plant is 30 to 40 cm tall: it then branches and gives many more flowers. Next, remove spent flowers regularly, cutting the stem back to the first bud or a leaf, to keep production going until the frosts.

The large varieties, especially the big flowered dahlias, have heavy stems that snap in wind and rain. Put in a strong stake right at planting, before the clump develops, and tie the stems in as they grow.

Pests and diseases

Slugs and snails are the first enemy, formidable in spring on the tender young shoots they can devour in a single night as they emerge from the soil. Watch closely at this stage, protect the young plants and gather them in the evening after rain.

Late in the season, powdery mildew often appears, a white felting on the leaves favoured by the humidity and temperature swings of late autumn. Open up the clumps well, water at the base and remove affected leaves. Aphids can also colonise the young shoots and buds, to watch for in early summer.

Propagation

Dividing the tubers at the end of storage is the simplest method. A few weeks before planting, split the clump into pieces each bearing a section of crown with at least one visible bud (an eye), because that is where the new stem starts. A piece with no eye will give nothing.

Taking cuttings is possible to multiply a fine variety: start a tuber in warmth six to eight weeks before your planting date, and take the young shoots of 5 to 8 cm to root them. Growing from seed also exists but does not reproduce varieties faithfully, it is mostly reserved for dwarf dahlias from mixtures.

Month-by-month care calendar

Green Hub tips

• Never plant before the last frosts have passed: the dahlia is frost tender and the slightest late frost destroys the tuber. Allow two to three weeks of margin, the time it takes for the soil to warm up.
• In autumn, wait until the frost has blackened the foliage, then lift the tubers, clean them and keep them dry and frost free in a crate of peat or dry sand.
• Pinch the main stem when the plant is 30 to 40 cm tall: you will get a branched clump and many more flowers.
• Remove spent flowers as you go, the flowering restarts and carries on until the frosts.
• Put the stake in right at planting and protect the young shoots from slugs, which are greedy on dahlias in spring.