Dahlia Blue Boy

Dahlia ‘Blue Boy’ is a classic favourite for anyone who loves rich, jewel-toned flowers. Like all “blue” dahlias it isn’t truly blue—think deep violet-purple with a blue sheen, giving a velvety, dramatic look that stands out beautifully in the summer border. The blooms are produced freely from midsummer right through to the first frosts, and the colour is perfect for pairing with pinks, apricots, whites and silvers (or going full drama with reds and oranges).

It’s also a brilliant cut flower, with strong stems and long-lasting blooms in a vase. Plant it in a sunny spot, keep it watered and fed, and you’ll be picking flowers for weeks on end.

Planting Instructions

When to Plant:

Plant Dahlia tubers in spring. For the earliest flowers, start tubers in pots indoors from March to April, then plant out once the risk of frost has passed. If planting directly outside, wait until late spring when the soil has warmed.

Where to Plant:
Choose a sunny, sheltered position with fertile, well-drained soil. Dahlias are ideal for mixed borders, cutting gardens and patio containers. They love warmth and perform best in full sun.

How to Plant:

• Depth: Plant tubers about 10–15 cm (4–6 inches) deep, with the old stem (or growing point) facing upwards.
• Spacing: Space plants 45–60 cm (18–24 inches) apart, depending on the final size.
• Soil Prep: Enrich the planting area with well-rotted compost or manure. In heavy soil, improve drainage with grit.

Watering:
Water after planting. Once shoots appear and growth is underway, keep soil evenly moist, especially in dry spells. Avoid waterlogging. Container-grown Dahlias may need watering daily in hot weather.

Aftercare Tips:

• Pinch out the growing tip once plants are around 30 cm tall to encourage bushier growth and more flowers.
• Stake taller stems if needed, especially in exposed borders.
• Deadhead regularly to keep plants flowering until autumn.
• After the first frost blackens the foliage, lift tubers and store dry and frost-free over winter, or protect in the ground with a thick mulch in milder areas.

Top Tip:
If you want loads of flowers for cutting, start picking early and keep deadheading—Dahlia ‘Blue Boy’ will reward you with fresh blooms right up to the first frosts.

Plant Details

Flowering Period:

July - October (until first frosts)

Height:
36–48" (90–120cm) (approx. depending on conditions)

Position:
Full sun

Tender Perennial:
Overwinter tubers frost-free in the UK