Grow Dahlias From Seeds Or Tubers

Last week we began a journey into the world of dahlias with a bit of their history and ending with a look at the basic dahlia groupings and flower types. This week, as the tubers show up in local garden centers, some thoughts on growing dahlias from seed and some of the ins and outs of tuberous dahlia culture. But first, did you know that dahlias are actually perennials? In their native habitat and even in the southern states, they never need to be dug. They simply die back to the ground in the dry season and re-emerge the following spring.

If you are going to try to grow dahlias from seed (these will be bedding dahlias and not the taller tuberous dahlias) you’ll find them relatively easy, but you’ll need to get started very early in the spring or in late winter. Seed should be sown indoors around mid-March and will take 10 to 14 days to germinate, or seed can be sown outdoors (a riskier proposition) when the soil temperature reaches 65 degrees. The outdoor sowings will result in late-blooming plants that will provide great fall color. And if you are after the large flowered tuberous types from seed, it can take up to two years of growth to get any flowers. The seed is large and two to three seeds should be planted in holes about 1-half-inch deep or into peat pots.

Most of us, however, will choose to buy dahlia tubers through the mail or at garden centers and many will stick to the smaller potted or cell pack bedding dahlias. Before you place your order or head to the garden center in May, take a moment to consider where you’ll be doing your planting. These plants give their best performances when planted in full sun in a moist, well drained garden loam. If you have a spot that may have heavy clay in it you can prepare the ground by working in liberal amounts of well-rotted manure, peat moss or compost to a depth of eight inches.

A time release fertilizer or an organic fertilizer can be worked into the soil at planting. No further fertilizer containing nitrogen should be added during the growing season or you are likely to be left with wonderful, lush, green plants with few if any flowers.

In planting the tuberous roots, dig holes 4 to 6 inches deep and wide enough to accommodate the tubers, which may be as long as 6 inches. Your planting hole should be as wide as the tubers are long, since most tubers will be planted both long and deep. Place the tuber flat in the hole in a horizontal position with the eye or bud facing upwards.

When planting taller varieties, it’s advisable to put a 4-foot garden stake about 1 inch from the end of the root bearing the bud. The young plant can thus be given support all the time until it reaches full growth. The danger in waiting to do the staking until after the shoot emerges from the ground is that you are likely to push the stake through the tuber. Another important thing to remember in staking is the color of the stake. A natural bamboo stake will stick out like a tall dead stem against the dahlias green shoot so medium green staking is preferable and Takiron stakes work well.

Once the tuber is in the hole, cover it with enough soil to make the depression level again. Not more than one tuber or root should be planted in a hole. This allows the plants to grow and spread, allows air to circulate among the plants and provides for easy cultivation. Outdoor planting can begin around mid-May and for earlier blooms you can start the plants in large pots in a sunny indoor location. But there is a trick to getting great dahlia blooms well into the fall. This is accomplished by delayed planting and you achieve this by storing in the refrigerator some sproutless tubers in sealed plastic bags filled with dry peat moss. These tubers can be started in large pots (2-gallon size or larger) in mid-June and then transplanted to the garden (and staked) as bare spots become available.

As the garden plants grow, they should be tied to the stake using either twine, coated wire ties, sisal or stretchable plastic ties by making figure-eight knots so that the stem is supported, but free to move and not snap in strong winds. The plants can be cultivated about once a week eliminating weeds and keeping the soil in good condition, though they do best when the soil is lightly mulched. When the plants are in flower and spaces between them are filled with foliage the need for weeding should diminish. But the best way to keep the weeds down is to add a mulch of seedless straw or another weed-free mulching material around the plants, but try to keep the mulch about an inch from the stem.

Dahlias are generally pest free. Although aphids or the dahlia stalk borer may show up, they are rarely a problem. The best way to keep these problems at bay is to rotate these plants from year to year so they are in different garden locations. Planting them in the same spot every summer simply encourages these insects to remain in the same spot, almost guaranteeing that you’ll end up with problems such as dahlia mosaic, which can be spread by aphids and possibly thrips. One summer I had a horrible time with two-spotted spider mites. When it’s hot and dry these insects proliferate very quickly and your best defense is an early warning system. Every week look on the undersides of the foliage with a 10x magnifying glass (also known as a gardener’s loupe). If you spot the mites, immediately begin spraying the undersides of the foliage with a hard but fine spray of water every three days. About two weeks of this routine will knock them out without the need for insecticides.

For gardeners with time to spare or who want magnificent, exhibition quality flowers, there are a few simple pruning and disbudding techniques to learn. For very large blooms (the variety must be a large flowering type to start with) prune the plant to a single main stalk by pinching out the side shoots. When flower buds appear, remove the two lateral buds in the main cluster leaving only a single bud. The tall plants reaching 4 to 5 feet will need support, such as stakes with string around the plant to keep these giant blooms upright.

When cutting dahlia flowers it’s almost a necessity to do your cutting as early in the morning as possible before the sun strikes the plants. The stems are hollow and searing them with a match or lighter also helps in keeping them turgid. The next step is to remove any excess foliage and then plunge the stems into cold water.

Don’t be surprised if your plants bloom only sporadically in the dog days of August. As soon as the cooling breezes hit the Hamptons after Labor Day, they’ll perk right up and continue to flower until mid- to late-October. After the first frost the tubers can be dug and stored just like canna tubers in peat moss or sand in a cool (not freezing), dry basement or garage. Keep growing.