How to plant dahlia tubers outside

I’m going to show you how to plant dahlia tubers. If you’ve overwintered tubers or bought new dahlias, wait until the risk of frost has passed before planting them outside. I’ve been nurturing this tuber in a pot in my greenhouse, and it’s already put on plenty of vigorous new growth.

Prepare a pot. You can plant a tuber in a pot or in the border. I’m using a large, clean pot with a drainage hole, loosely covered with a crock or an old plastic pot so it doesn’t block. I mix Richard Jackson’s Easy Compost with a little grit and vermiculite. Perlite or sand also helps make the compost free-draining.

Plant the tuber. Fill the pot about three-quarters full with peat-free compost and make a well. I’m adding Richard Jackson’s Root Booster, a slow-release feed with humates and mycorrhizal fungi to help the plant take up water and nutrients. Set the tuber in with the shoots pointing upwards, around 10–15 cm deep, depending on the variety. Large dinner-plate dahlias can go a little deeper for stability. Fill in with compost and firm it gently.

Water the pot well. Pots are difficult to keep moist in summer, and very dry compost can repel water. I mix 10 ml of Richard Jackson’s Bioactive Wetting Agent into a 4.5-litre can of rainwater and soak the compost so the water reaches the roots. I reapply it every two weeks, or weekly in drought conditions.

Pots dry out faster than plants in the border. Give the compost a long soak whenever it feels dry. That might mean three or four times a week, or daily in hot weather. Short, shallow sprinklings encourage shallow roots. Water at the base and keep the leaves dry to help reduce fungal problems such as powdery mildew.

With so many dahlia varieties, there’s one to suit almost any planting scheme. Get planting and enjoy a summer of colour.