Information from the American Dahlia Society and Swan Island Dahlias
Container: Use a heavy-weight, sturdy, clean plastic pot with good drainage and a diameter of 16-20 inches. Miniature dahlias can be contained in a smaller pot, with a diameter of about 10-12 inches.
Soil: ADS recommends using a coarse soilless mix, like Ball's Growing Mix #2 (blend of fine bark, vermiculite, peat, and perlite). Swan Island Dahlia, on the other hand, recommends using two parts garden soil (plain dirt, not treated, nothing added) and one part potting soil that has not been treated in any way. Dahlias need as much plain dirt in these containers as possible. Do NOT use pre-fertilized potting, as it will burn them.
Planting the tuber: Place a biodegradable drip-coffee filter (or two, if necessary) over the holes on the bottom of the flower container in order to keep the soil in and bugs out. Fill the flower pot 1/3 full with pre-moistened potting soil. Lay the tuber horizontally on top of that layer of soil. If at all possible, place the tuber so that the eye end will be in the center of the pot. If a sprout is already growing out of an eye, place the tuber so that the sprout is pointing upwards. Cover the tuber with more pre-moistened soil, but just enough soil to hide it. The eyes may be exposed, if desired, to watch for growth. Using a spray bottle filled with warm water, mist the tuber until the surface is damp. Do NOT fill the container to the top with potting soil at this time. By just covering the tuber, the plant's growth can be easily watched and overwatering of the awakening tuber can be prevented. As the stalk grows, carefully add more potting soil to the container, so as to not break the stalk from the eye of the tuber. Do not cover the upper set of leaves.
Winter: Most dahlia growers in our area (Northeast Virginia) dig up and store their tubers in a cool, dark area, like a garage, for the winter. Some put the tubers in plastic ziplock bags, covered with vermiculite, and store them in a Rubbermaid container in their garage. Garages in our area typically stay between 45 and 55 degrees Fahrenheit in the winter, which keeps the tuber safe. However, don't put too many in one container, as they are more likely to rot.
Stakes: Tomato cages, metal rods, or bamboo stakes help to prevent the stalks and stems from breaking off in the wind or if/when the pot falls over. If the average height of the variety is known, use that length of stake. For large/medium dahlias, 6 ft stakes usually work the best. The plants can get well over 5 ft tall, and the stake has to be driven into the ground a foot or so. A good option for stakes are Home Depot's green plastic stakes, which are sturdy and better-looking than other alternatives. Occasionally, two stakes may be needed if there are two main stalks growing from the tuber. Tie the stalk(s) to the stake(s).
After being staked, when the plant has grown taller than the top of the container, add the rest of the potting mix to within one-inch of the top rim of the container. It is okay to cover the leaves below the soil line.
When the plant gets at least three or four sets of leaves, pinch out the growth tip of the stalk. Pinching helps to make a bushier, sturdier plant. It does not delay the blooming time of the plant, but the plant does make more flowers.
After being staked, when the plant has grown taller than the top of the container, add the rest of the potting mix to within one-inch of the top rim of the container. It is okay to cover the leaves below the soil line.
When the plant gets at least three or four sets of leaves, pinch out the growth tip of the stalk. Pinching helps to make a bushier, sturdier plant. It does not delay the blooming time of the plant, but the plant does make more flowers.
Tying: It is best to tie the dahlias to the stake twice a week. This may not need to be done to each one every time, but this schedule will help prevent them from toppling over in inclement weather. Green gardening string is a good choice, as it blends in well with the plant and is cheap and easy to use.
Watering: Make sure to keep your soil slightly damp, or cool, to the touch after planting, not soaking wet. After plants are 12 inches high, potted dahlias will require extra watering and fertilizing to promote proper growth and blooming. While the plant is developing roots, let the soil almost dry out before watering again. Water the plants after adding more soil to the pot.
Fertilize: Dahlias require a low nitrogen fertilizer with a high percentage potassium and phosphorus such as a 5-10-10, 10-20-20, or 0-20-20. Look for a fertilizer where the first component number is 1/2 of the other two numbers. Usually a bloom food or vegetable type fertilizer.
Pests and Problems: Spray with the combination fungicide and pesticide (ex. Orthene III) in the early evening (if the air is still and cool) at the first sign of powdery mildew, spider mites, or thrips. In August, outer green leaves get thinned out to allow the inner branches to receive light so they can grow and make more flowers.
The best thing about container gardening is that the plants can be moved! This way of growing container dahlias is not written in stone. This is a starting point to help develop your own system of growing contained dahlias.
If you are growing potted dahlias on a cement patio, put the container in the shade during the hottest part of the day or the plant could get too hot and burn from the reflected heat of the cement. Morning sun is the best, especially if you live in a hot summer area.
For information on a dahlia cut flower trial done by the Association of Specialty Cut Flower Growers, visit this link.
If you are growing potted dahlias on a cement patio, put the container in the shade during the hottest part of the day or the plant could get too hot and burn from the reflected heat of the cement. Morning sun is the best, especially if you live in a hot summer area.
For information on a dahlia cut flower trial done by the Association of Specialty Cut Flower Growers, visit this link.