After bringing us a summer of glorious colour, dahlias now need our help to survive winter. You probably planted your dahlias as tubers in spring. These tubers can now be saved for replanting next year.
Dahlias are not truly hardy – if the growths or the tubers are frozen they will die. The active part of the tuber, where new shoots grow, is at the base of the old stems, at the top of the tuber. If winter frost reaches these areas the plants will be killed. You therefore have two options: you can leave the tubers in the ground or you can lift them and protect them.
In many gardens the tubers will survive if left in the garden over winter. You can help by putting a loose mulch over the plants (such as bark). However, if the winter is very cold the tubers may not survive and slugs and snails as well as winter wet can cause damage to the tubers and spring growth. Plants also become large and shoots will be overcrowded if left in the ground and lifting also give you the chance to divide the tubers and make more plants.
If you decide to lift and store the tubers, you can then start them into growth in spring and you can not only divide them, to make new plants, your plants will bloom earlier and for longer.
Once the foliage has been ‘burnt’ by the first frost, or before if you have not had a frost, cut back all the growth to about 15cm high. The stems can be composted.
You then need to dig up the tubers. You will find that the tubers are huge compared to what you planted! So push in a fork well away from the stems and lift the tuber carefully to avoid damaging the individual roots.
If you do damage a fleshy root it is best to cut it off because it will rot. This will not matter too much because the new shoots will grow from the top of the tubers, at the base of the old stems and not from these actual roots. Pull and shake off as much soil as possible so the roots are clean. Then allow them to dry off and shake off any more soil.
You then need to store the tubers. The key points are that the tubers should be cool so they do not sprout yet, that they are dry and that they are free from frost. I tend to store them in trays, loose under the greenhouse staging. Some people wrap them in newspaper in the shed. Others string them up and hang them in the garage. You need to check on them to make sure they are not getting too dehydrated or that they are rotting. Never wrap them or store them in plastic.
Do not cut the tubers yet unless essential – it can lead to rot on the cut surfaces. You can cut and divide the tubers in spring if you want.
You are then set for an even better display of dahlias next summer – something to look forward to.
