How to Grow Dahlias
Dahlias love sunshine! A spot with at least 8 hours of direct sunlight is ideal. However, in very hot climates, dahlias are better planted where they receive morning sun and afternoon shade. They should be planted between mid-April and May, around the same recommended local time to plant a vegetable garden. The soil temperature should be around 60 °F/ 16 °C, with no more chance of frost.
If you have heavy soil, amend it with compost. Dahlias love it if you add a handful of bone meal to the planting hole. They prefer a pH of 6.5-7.0, so if you live in an area with acidic soils, you should also add a handful of lime.
To plant your tuber, dig a hole slightly larger than the tuber and 4 to 6 inches deep. The tuber should lay horizontally in the soil, with any visible buds pointing up. Do not water your dahlia tuber until it you see it coming up. Otherwise, it might rot. The exception to this is in very hot climates, where you should water them only a little. This is also a good time to protect dahlias from slugs, using whichever slug prevention means you prefer.
Once your dahlia begins growing, they will require deep watering every 2-3 days (more or less, depending on the weather). Dahlias should be kept well weeded, by hand only.
Dahlias do not require a lot of fertilizing, in fact too much and the plants will not grow as well. It is important to avoid fertilizers or composts that are high in nitrogen. The small amount of fertilizer that is used should be low in nitrogen but high in phosphorous and potassium, and other micronutrients. Fertilizing can be done within 30 days of planting and again about 4 weeks later.
The more cut flowers you harvest, the more cut flowers your plant will produce! Removing spent blooms will inspire the plant to continue to bloom. Pinching back new growth to the first leaf node will encourage a shorter, bushier plant, if that is preferred. Very tall plants may need to be staked.
If any pests show up, contact your local Master Gardener Program for regionally specific advice on how to protect your dahlias. They are typically susceptible to earwigs, cucumber beetles, mildew, spider mites, snails, and slugs.
Fall and Winter Care
In cold climates, it is best to protect your dahlia tuber in some way. Tubers have thin skin and will rot if they freeze or if they are too wet for long periods of time. It is good for the dahlias to experience one light, killing frost, so that the leaves can die back (turning brown) and allow the plant to transition into dormancy. Always cut dahlia stems completely to the ground after this point if not digging up. Some people plant their tubers alongside a house or structure, where absorbed sunlight and heat from the structure provides sufficient winter protection. Another option is to cover the area with a few inches of mulch. In wet areas, a layer of plastic under the mulch can provide additional protection from rotting. Digging and storing is the only way to be completely sure that your dahlia will survive. Where the ground freezes 4-6 in/ 10-15 cm deep, it should definitely be dug out. Pop the root system out of the ground with a shovel or digging fork and shake the soil off. Store them somewhere between 40-50 °F/ 5-10°C. They are best kept in boxes or crates filled with slightly dampened wood shavings, sawdust, peat moss or similar. Do not store in sealed containers or bags, otherwise they may rot.
Dahlias can be divided in the fall or spring. It is best to divide them every 1-3 years. An easy way to divide dahlias is to split the root center into halves or quarters, keeping in mind that new growth comes from eyes located where the tubers connect to the stems. Then you have more to plant or share!