Best Tips For Growing Dahlias

Follow our hints and tips for a late summer blast of colour from dahlias

Pre-planting

• They are grown from perennial tubers started into growth in early spring; except the dwarf varieties which can be grown from seed.
• In March place the tubers in a shallow box in a compost mixture of equal parts peat free compost and sharp sand, or John Innes No 2. Discard any tubers which are soft or showing any sign of mould.
• Don’t cover the crowns with compost and take care not to water into the crown otherwise they could rot.
• If there are a lot of tubers (over 4) in one clump they can be divided with a sharp knife. Don’t cut into the actual tuber. Dust the cut edge with yellow sulphur. Splitting them regularly maintains their vigour.
• Keep just damp at a temperature of 15 – 18C (59 – 64F).
• Cuttings can be taken from the tubers which will increase your stock, but generally flower 2 - 3 weeks later than tubers. Once the new shoots have reached at least 8cm (3”) cut off with a sharp knife leaving 0.5cm (1/4”) attached to the crown. Remove the lower leaves and dip the cutting in hormone rooting powder. Insert 4 cuttings around the edge of a 9cm or 3” pot, in a compost mix of equal parts peat free compost, loam and sand, or potting compost.
• Keep cuttings damp and out of direct sunlight in a frost free environment. They should have rooted in 3 – 4 weeks, after which they want potting up singly in 9cm (3”) pots.
• Start hardening off the cuttings or tubers in April by leaving the cold frame open during the day or taking outside and bringing them back in at night.

Planting

• Plant outside after all danger of frosts has passed.
• They require a sunny position in a well-drained but moisture retentive soil. They flower best with between 6 – 8 hours of sunshine a day.
• If you don’t have anywhere frost free to start them early they can be planted straight into the ground after all danger of frost has passed; they just won’t flower as early as if started off in March.
• Enrich the planting hole with well-rotted farm yard manure or good quality compost.
• Plant the dwarf varieties 8cm (3”) deep and the large growing varieties 12cm (5”); planting too deep reduces the plant’s vigour which results in fewer flowers.

Aftercare

• All, except the dwarf varieties, need staking; either an individual stake per plant or a circular plant support around a clump of plants.
• If you want a bushy plant with a lot of flowers you need to pinch out the growing tip and the growing tips of any subsequent side shoots; if you want a taller plant leave the pinching until a little later.
• If you are growing exhibition varieties pinch out any side shoots, leaving just one bud per plant.
• The more flowers the plant produces the smaller they will be.
• Water well especially in hot weather.
• Feed with a balanced feed until the first buds appear then switch to a high potash feed, such as Tomorite.
• It is essential to keep deadheading, or picking for cut flowers, to encourage the plant to keep on producing more flowers. Cut the flower stem just above a pair of leaves, and new buds should form in the leaf axils.

Problems

• They are susceptible to all the usual garden pests including: aphids, red spider mite and thrips. They are also infamous for attracting earwigs.
• A severe infestation of aphids results in distortion of the young growth and weak growth due to the loss of sap, so you need to be vigilant and use an insect control at the first signs of any insect damage. Aphids can also transmit viral infections.
• Feed the birds and they will feast on aphids.
• Earwigs can be controlled by placing an up-turned pot stuffed with straw on the top of a cane.
• Slugs can also be a problem so protect as soon as you plant them out.
• Mildew is also a problem but can be prevented by ensuring a good air-flow around the plant, removing any dead foliage and making sure it doesn’t dry out.

'Bishop of LLandaff'

Varieties

• Anemone – a double flower growing to a height of 60 – 105 cm (24 – 42”), with a flower up to 10cm (4”) across; they need planting 60cm (24”) apart.
• Ball – these are fully double forming a ball; they also come in miniature.
• Bedding and dwarf hybrids – flowers vary from single through to double and grow to a height of 30 – 60cm (12 – 24”) with flowers up to 7.5cm (3”) across; they need planting from 30 – 60cm (12 – 24”) apart.
• Cactus and semi-cactus – fully double with a rolled, pointed petal. They come in giant, large, medium, small and miniature; flower size and planting space depends upon variety chosen.  
• Collarette – a double flower, often two colours, up to 10cm (4”) across, growing to a height of 75 – 120cm (30 – 48”); they need planting 60 – 75cm (24 – 30”) apart.
• Decorative – these have many petals and no central disc, coming in giant, large, medium, small and miniature sizes. Planting space depends upon the variety.
• Paeony – many petals growing to a height of 90cm (36”) with a flower up to 10cm (4”) across; they need planting 60cm (24”) apart.
• Pompom – similar to the ball, growing to a height of 90 – 120cm (36 – 48”) with blooms up to 7.5cm (3”) across; they need planting 75cm (30”) apart.
• Single – a single outer ring of petals with an inner central disc. They grow to a height of 45 – 75cm (18 – 30”) with flowers up to 10cm (4”) across; they need planting 60cm (24”) apart.

Storing

• Wait until the first frost has blackened the foliage before lifting the tubers.
• Cut the stems 15cm (6”) from the crown and leave to dry out for several days.
• Once the soil is dry brush off, on no account wash the soil off as this will only encourage mould to grow.
• Trim the stems to 10cm (4”) and dust with yellow sulphur, to prevent any fungal growth.
• Place in a box and cover with sawdust or dry peat substitute to the base of the stems.
• Leave over-winter in a cool, dark, well-ventilated, frost-free spot away from any vermin.

For more hints and tips on how to achieve a colourful herbaceous border just get in touch with our Outdoor Plant team here in store.