If you received tubers that started growing long stems looking for light in your mailing package, do not fret. You can get two or three extra plants from your tuber by planting the stems separately, plus plant the healthy tuber. A tuber sprouting with growth is always good news.
Recently I found a box of tubers in peat that all looked long the spider Daddy Long Legs, But all of their legs were white. Each Dahlia tuber clump had sprouts/stems that grew the length of the box and back. All of that growth energy is not going to be wasted. I am going to use it to my advantage.
If you ever find yourself in this situation the best advice is to cut those stems off close to the base of the tuber where they originated. More sprouts will come from that same area. Wiggle off or cut with a sterile knife. If you were to plant with those leggy white stems you would not have strong straight stems. They are likely to flop over or be crooked from being bent one way and then growing at a different angle after planting. Plus the added weight of the flowers later could break the stems. So I cut these way back and save these foot-long sprouts.
You can now plant your Dahlia tuber as usual in prepared soil and water after planting if there is not much moisture in your soil. Often in the Spring the soil has plenty of moisture and I do not water after planting. The Spring rains do that for me, but I realize not everyone gets as much rainfall as we do in Maryland.
You are now left with those white leggy stems and what I do is cut the long ones in half and pot these all in a potting soil/compost mixture with coarse sand added and/or vermiculite. You want the roots to be free to grow and not be bound or sitting in “soggy soil.” Do not buy potting soil with fertilizer in it as this usually ends of burning them and your success rate won’t be so high. The stems are white but the portion above the soil will slowly green up in a few days with sunlight.
In approximately two weeks the stems should start growing roots under the soil. Don’t keep pulling the stem out to check as those roots can easily break off. When new growth shows and the leaves start to grow you will know they have rooted. Success. The roots will form from the leaf nodes under the soil. You can now plant them in your garden and make sure the soil is free draining and does not dry out.
The first Dahlia plants that grow from those stems will likely bloom before your tubers that you planted. A fun fact. They will also grow small tubers the very first season, possibly three to five small tubers before the growing season is over. A great way to increase your Dahlia collection.
If you give Dahlias a chance they are anxious to grow in the right conditions and will give you bouquets of all sizes and shapes of flowers. The diversity is delightful.