MONTY DON: It’s time to wake up your dahlias from winter hibernation

Back in the 80s, and even the 90s, there were only a few dahlias that were at all acceptable to self-appointed arbiters of horticultural good taste, but thankfully we have moved beyond all that nonsense and I’d recommend that every garden should grow as many dahlias as they have room for. I certainly do.

Dahlias were discovered in Central America by Spanish explorers in the 16th century and later introduced to Europe. There were attempts to establish them as a food crop, along with other New World imports such as potatoes and tomatoes, but it was quickly found that while dahlias were, strictly speaking, edible, they tasted horrible.

As a result, botanists focused on the plants’ decorative rather than nutritional qualities, breeding hybrids with more spectacular blooms than the species parents.

If you have very free-draining soil and live in the south, dahlias can be safely left outside, especially if they’re planted good and deep so they have an insulating layer of soil above them. 

Monty Don recommends to grow as many dahlias as you have room for and says they can be safely left outside if you have free-draining soil and live in the south

But I have found that it’s worth digging them up and bringing them in under cover over winter, otherwise I lose more than the number that make it through to spring.

The secret to storing dahlias is to cut off the stems with secateurs and then clean the tubers – removing any that are damaged and checking they’re not harbouring any slugs – before storing the tubers in a material that will insulate them and stop them drying out. 

In the past I’ve used spent potting compost, building sand, vermiculite and coir, but now it’s scrunched-up newspaper, which is a good way of recycling it and works extremely well. Then pack into crates and put them in a cool, dark but frost-free place; a cellar or garden shed is ideal.

However, it’s now time to bring them out of hibernation and prepare them for replanting. If you put them straight out into the border there’s a real risk that the new shoots will be nipped by a late frost, and although this is unlikely to rot the tubers, it means that you’re back to square one and flowering will be delayed.

It's time to bring them out of hibernation and prepare them for replanting, keeping them watered and then in a week or two, they will produce strong young shoots

It’s time to bring them out of hibernation and prepare them for replanting, keeping them watered and then in a week or two, they will produce strong young shoots 

It’s better that when you take the tubers out of their protection you start by checking them over, feeling them for firmness and discarding any that are either rotten or shrivelled. The latter condition – a result of being too dry – is more likely, and it’s a good idea to water the tubers once or twice in winter to keep the moisture levels up.

Then pot up the tubers in a recycled plastic pot, using fresh peat-free compost so that they can grow strongly for the next month or two with as much protection as they need. Plant them out when all risk of frost has passed and when they are strong, with 30-45cm of stem and foliage.

This is also the time to prepare a few plants to provide cuttings. Pot them up so the tubers are only just below the surface of the compost, water well and place the pot on a heated surface. If you do not have a heated mat, a windowsill above a radiator will do fine. 

Keep them watered and in a week or two, they will produce strong young shoots. When these are about 8cm long, cut the strongest where they join the tuber. Put these cuttings into compost mixed with an equal volume of grit or perlite and place somewhere light and warm.

Water well and spray with a mister at least once a day. They will root quickly and can be potted on individually after a few weeks, then grown on as young plants to provide late flowers this summer and healthy new plants next year. 

Monty's plant of the week this week is a dog's tooth violet (Erythronium)

Monty’s plant of the week this week is a dog’s tooth violet (Erythronium)

MONTY’S PLANT OF THE WEEK: Dog’s Tooth Violet (Erythronium) 

The petals of erythroniums are held on a tall stem and are reflexed back like a bonnet or Turk’s cap lily. The plants like light, leafy soil in dappled shade that does not get too hot or too dry in summer, and they’re ideal for growing underneath deciduous trees and shrubs. 

You can buy plants now or, in autumn, grow them from scratch from the long, dog’s tooth-shaped bulbs (hence the common name for the plant). Erythroniums are easy to divide and replant either immediately after flowering or, ideally, when the foliage begins to die back.

ASK MONTY

Q When is the best time to divide a Michaelmas daisy?

Roger Scott, S Wales

Monty said the best time to divide a Michaelmas daisy is in the spring when you start to see new growth

Monty said the best time to divide a Michaelmas daisy is in the spring when you start to see new growth 

AIn the spring, as you start to see new growth. Dig up the plant and tease the roots apart, either making a couple of substantial clumps or many small segments that can be potted up and grown on for planting out next spring.

Q My purple sprouting broccoli has clubroot. Is my veg plot now useless for growing brassicas such as this, and can I plant potatoes there this year?

Maggie Turnbull, Glos

AClubroot is a fungal infection that can affect all brassicas but potatoes will be fine. Adding lime to soil helps lessen the effects of clubroot, and ideally you should use a six-year rotation, meaning you don’t grow brassicas in the same ground for six years.

Q I need to move one of my roses. When should I do this?

Philip Tyrrell, Somerset

AIt’s best when the rose is dormant – showing no signs of new growth – usually December to March. Prune it back hard, dig it up and replant immediately, adding mycorrhizal powder to the roots. Water well and don’t let it dry out all year.

THIS WEEK’S JOB: SOW TENDER ANNUALS 

Sow the seeds of tender annuals in peat-free compost in a seed tray, allowing plenty of room for root growth, and place in a greenhouse or on a windowsill. Prick out the seedlings into small pots when large enough to handle, and slowly harden them off before planting out in late May or June. 

Monty said that this week's job is sowing tender annuals in peat-free compost in a seed tray, allowing plenty of room for root growth, and place in a greenhouse or on a windowsill

Monty said that this week’s job is sowing tender annuals in peat-free compost in a seed tray, allowing plenty of room for root growth, and place in a greenhouse or on a windowsill