Dahlias are the quintessential flower for late summer blooms. They happily bloom continuously once they begin in late July or early August in my zone 5b garden. Be warned however, while they are not internally fussy, there are some tricks of the trade to ensure they are happy in your garden space.
What is a tuber
Dahlias are grown from tubers. A tuber is part of the stem system of the plant that grows underground, it's root system will grow out of the tuber. To know if your tuber is viable (or if it will grow into a plant), you need to find eyes on the old part of the stalk (the very tip of the tuber). From the eye, an entire plant will grow.
Inspect all newly purchased tubers carefully, to make sure you've received something that has the ability to grow. Sometimes, eyes have already begun to sprout, making them extremely easy to find. Other times, they have yet to sprout, and you'll have to look very carefully for bumps on the stalk. Given time to warm up, they'll sprout too.
Purchasing Tubers
Tubers can be bought online through commercial growers, big box stores, or at your local garden centre. They should be purchased so that you have them in time for spring planting. There are pros and cons to buying from all avenues. Big box stores, and greenhouses typically have a small assortment of imported from Holland tubers. On a positive note, you have the ability to get them in hand quickly to get a jump on the growing season. Commercial growers have a wide assortment of domestically grown tubers, but generally do not put them in the post till the danger of freezing has passed in April (this is true for Canadian Climate). Domestically grown tubers have a lesser chance of introducing soil diseases such as leafy gall, or root gall, although it can still happen.
Determining Quality
TUBER SIZE DOES NOT DETERMINE QUALITY. All dahlia cultivars produce tubers a little differently, in size, shape and storing ability. Some of my cultivars that produce tiny tubers (Apricot Desire for instance) make tiny round tubers, and yet they are undoubtedly floriferous.
Some tubers store differently than others. Some wrinkle, some stay perfectly plump. If there is an eye, then you'll get a plant.
How to Plant
When you get your tubers, depending on the date, you can chose to start them indoors in a pot, to transplant out when the danger of frost has passed. This allows you to get a jump start on the season, especially useful in colder climates. You can start them indoors 4-6 weeks prior to last frost. You can also chose to plant directly into the ground after your last frost date.
I like to end my season with a soil test in November. This was you can see where the soil is depleted and what amendments you can make in early in the spring. One of the most important factors is your pH level in your soil. pH determines the availability of up take of nutrients in your soil. Dahlias like a pH of 6.3-6.8 typically.
Dahlias can be planted out when the soil temperature is reliably above 15 degrees Celsius. Personally, in my field, I directly plant in to the ground, as I do not have the space to start tubers indoors.
Dahlias should be planted at a depth of 10-15 cm. I tend to dig a little deeper and amend with compost directly in the planting hole. They should be spaced 30 cm apart. This information can slightly differ between dahlia cultivars, some are smaller plants and don't require as much spacing, or don't produce deep tubers, so can be planted shallower. This does stand as a good overall guide.
Unless your soil is very dry, and there is no chance of rain, refrain from watering your tubers until their sprouts have broken the ground. They are susceptible to rot otherwise. After they have emerge, they benefit from regular watering between rains.
I fertilize one month after the dahlias emerge from the soil. It's best to use a fertilizer that is low in nitrogen, and balanced in phosphorus and potassium.
Pinching
When your dahlia is about 4 inches tall, I recommend pinching and removing the inner stem. This will force side shoots, and you'll have a bushier plant. You'll start to see your first buds form in July.
Dead heading for continuous blooms
It is important to continually remove dead blooms, in order for the plant to keep producing flowers. If the dead flowers remain on the plant, it signals that it's life cycle is complete, and will no longer produce new buds.
Storing
Enjoy your dahlias for the summer, and after the first hard frost hits, your dahlia plants will turn black and die off. In warmer climates, dahlias can be dug up after at least 120 days of growing.
At this point, you'll have to dig up the tubers, and store them at approximately 3-5 degrees C until spring, when you'll be ready to do plant them all over again!