An Easy Way to Overwinter Dahlias

The traditional way of storing Dahlias overwinter is to dig them up and  then dry out the tubers, usually packed in bone dry compost or vermiculite, see below.

However, this method is not easy. Very often, it is difficult to eliminate moisture from the tuber and  which then tends to get mildew and rot.

The reason we lift the tubers is that in colder, wetter parts of the country and those with heavy lay soils which are waterlogged, the tubers will rot in the garden bed over the winter. If you garden in an area which is predominantly drier, like the eastern UK, or if you have light sandy soils without severe frosts, you can safely leave Dahlia in the ground all winter.

New Growth on overwintered Dahlia

Elsewhere, as an alternative to lifting, grow Dahlias in containers. Illustrated left is a Dahlia grown and overwintered in a container, and it's full of lush new spring growth.

When the season ends and frost blackens the foliage, cut it back almost to the soil level. Mulch well. You can then decide where to place to container for the winter. In cold areas, consider placing in a greenhouse, lean to or the shelter of a wall or shrub to fend off the worst of the winter cold.

Keeping the tubers on the dry side is important. In wet areas, place the container in a sheltered spot from rainfall. 

In early spring, around March time, dig out much of the old compost right down to the tubers and re fill with good quality peat free compost and place the container in a nice sunny spot where you would like it to flower. Periodically, you may need to remove some tubers as the plant matures to prevent the container from becoming overcrowded. Dahlias grown this way in containers will always benefit from feeding with a general purpose or potassium rich feed such as tomato food. 

This is much easier than lifting the tubers and, as shown in the image, excellent growth re appears. Gardeners often use this method of overwintering in show gardens where conditions are not favourable for leaving the tubers in the ground. If you inspect some of the classic herbaceous borders in large stately gardens, you may see the rim of a container just showing in the soil. The gardener placed the Dahlia in a container for overwintering, then buried the entire pot in the ground at soil level for it to grow and flower over summer. They later removed the entire container in the Autumn.

The traditional way to overwinter Dahlias

The traditional way to store and overwinter is to dig and lift the plant. Cut off all the top growth, so you have just stem and tuber. Take care when digging up the tuber not to put the fork through the tuber or damage it as a cut or bruise can lead to rot when storing the tuber overwinter. Throughly clean off all clinging soil. This is best done by hand to ensure the tuber is clean for storage. Place the Dahlia in frost free place to dry out the moisture any retained in the tuber, which could take up to 3-4 weeks.

Once the tuber is fully dry, place the tuber in a box and cover the stem with vermiculite/sand/or dry compost to keep dry and frost free over the winter. It's a good idea to label them. If you are lucky, the Dahlia tubers will survive without mildew, saving you the cost of buying them again next spring. Check regularly and if any are showing signs of rotting, discard. It is not unusual to lose some tubers over the winter. If the winter is very wet, just the moisture in the area may cause a problem. Replant the tubers in March.