Dahlia

Value to bees

Nectar

High

Pollen

High

As a honey crop

Feeds the colony but rarely yields a surplus

Which bees, and why

Single dahlias — 'Bishop of Llandaff' and the collarette and anemone-flowered types — are worked constantly by honey bees, bumblebees, hoverflies and butterflies right up to the frost. Double dahlias are visited by nothing, because there is nothing there.

In single forms, completely open florets with exposed nectar and pollen. In doubles, the reward has been bred out entirely.

Worth knowing

Good for both in single-flowered forms across a very long late window, and close to zero in the doubles. The ratings here describe the single forms only.

Flowering

In flower

July to November (northern hemisphere)

Very long — from midsummer until the first hard frost blackens the foliage, which in a mild autumn can be into November.

When nectar flows

In single forms, generous nectar and abundant pollen from an open central disc. In fully double forms, essentially none.

Year-to-year variation

Dependable and entirely dependent on which cultivar was planted.

Growing it

Conditions

Full sun, rich moist well-drained soil, and generous feeding and watering. Frost-tender.

Size

40-150 cm depending on variety.

Native range

Mexico and Central America.

Suits

Small gardens, Large gardens, Containers and window boxes

• Buy single, collarette or anemone-flowered varieties if the aim is bees. The flower must have a visible open centre.
• Deadhead relentlessly and it will flower until the frost.
• Lift the tubers after the first frost in cold areas, or mulch heavily and leave them where winters are mild.
• This is the single best plant for demonstrating the double-flower problem to anyone who doubts it — grow one of each side by side and watch.

What it looks like

Tuberous perennials with dark divided leaves, carrying flowers that range from single open daisies with a visible yellow centre to fully double balls with no centre at all.