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A dahlia tuber has no roots when you plant it and cannot take up water. It sits in the ground living on its own reserves until the eye pushes a shoot up and roots follow. In cold ground that wait stretches to five or six weeks, and a dormant tuber sitting for six weeks in wet soil at 50F (10C) is simply food for rot fungi. In soil at 60F (15C) the same tuber is up in two to three weeks and away.
The trigger to plant is soil that has warmed to at least 60F (15C) at planting depth and the last frost being past or close. Dahlia top growth is killed outright at 32F (0C), so there is nothing to gain from having shoots above ground early. If your spring is long and cold, start the tubers in pots under cover four to six weeks ahead and plant them out as rooted plants once the frost risk is gone.
Choose an open site with at least six hours of direct sun and soil that drains. Dahlias are heavy feeders and heavy drinkers, but they will not tolerate sitting water at any point in their lives. On a heavy clay bed, plant on a slight ridge or work in a bucket of compost per plant rather than digging a deep hole that becomes a sump.
Do not water a newly planted tuber. It has no roots to drink with, and added water only feeds rot. Water once for the first time when you can see a shoot above the soil, and not before.
Depth, orientation and spacing
Dig a hole about 8 in (20 cm) deep and wide enough that the tuber clump lies flat without bending. Set the clump on its side with the eye or the old stem stub pointing upward, then cover with 4 to 6 in (10 to 15 cm) of soil. Too shallow and the plant blows over in summer; too deep in cold ground and the shoot exhausts itself before it reaches the light.
The eyes are the only part that matters. They sit on the crown, at the point where the tubers join the old stem, never on the fat end of a tuber. A tuber snapped off without a piece of crown attached will never grow no matter how plump it looks, so check for a small pink or white bump at the neck before you plant.
Space full sized varieties 24 to 30 in (60 to 75 cm) apart and compact bedding types 12 to 18 in (30 to 45 cm). This looks absurdly generous over bare soil in spring and is exactly right by midsummer, when each plant is a shrub 4 ft (1.2 m) across. Crowded dahlias mildew, hide their lower buds and drop far more flowers to earwigs, because the pests move plant to plant without ever crossing open ground.
| Item | Full sized types | Compact bedding types |
|---|---|---|
| Planting depth to top of tuber | 4 to 6 in (10 to 15 cm) | 3 to 4 in (8 to 10 cm) |
| Spacing between plants | 24 to 30 in (60 to 75 cm) | 12 to 18 in (30 to 45 cm) |
| Soil temperature to plant | 60F (15C) at depth | 60F (15C) at depth |
| Stake needed | Yes, driven in at planting | Usually not |
| Weeks from planting to shoot | 2 to 3 in warm soil | 2 to 3 in warm soil |
| First watering | When the shoot shows | When the shoot shows |
Drive the stake in before you backfill
A mature dahlia carries several pounds of wet foliage and flowers on hollow, brittle stems, and one summer thunderstorm snaps a plant off at the base. The support has to be in place before it is needed, and the only moment you can install it without damage is planting day, when you can see exactly where the tuber lies.
Use a stake of at least 5 ft (1.5 m) for full sized types, driven 12 in (30 cm) into the ground, set 3 to 4 in (8 to 10 cm) to one side of the tuber. Backfill around both together. A stake pushed in later, in midsummer, when the crown is invisible under a foot of leaves, goes straight through the tuber clump about half the time, and the wound rots in storage.
Tie the main stem loosely to the stake as soon as it reaches 12 in (30 cm), then again every 12 in (30 cm) as it grows. Use soft string in a figure of eight so the stem is not pressed against the wood. For a bed of several plants, three corner posts and two rings of string around the whole block work better than individual ties and are far quicker.
- Check the tuber has an eye on the crown Look for a small pink or white bud where the tuber joins the old stem. No crown, no eye, no plant. Discard tubers that are soft, shriveled to a thread, or moldy at the neck.
- Dig 8 in (20 cm) deep and wider than the clump Loosen the base of the hole and mix a spadeful of compost into the soil you removed. Do not put concentrated fertilizer in the bottom of the hole where the new roots meet it first.
- Drive the stake before the tuber goes in Set it 3 to 4 in (8 to 10 cm) off center and 12 in (30 cm) into firm soil. Doing this now is the whole reason the plant survives late summer gales without a broken stem.
- Lay the tuber flat with the eye up Cover with 4 to 6 in (10 to 15 cm) of soil and firm it gently with the flat of your hand. Do not tread it down hard, and do not water.
- Mark the position and leave it alone Push a label in at the spot. Three weeks of bare soil is normal, and every year people dig up a perfectly healthy tuber to see whether it is doing anything.
Record the planting date, the soil temperature and the exact position of each plant in the bed. Next spring you will want to know which spot came up first and which one never came up at all, and by then the bed is bare soil again with nothing to tell you.