Dahlias 101

Dahlias come in so many different colours with an abundance of interesting textures and shapes from formal to whimsical. You will find many varieties that suit your tastes whether you are looking to fill a cutting garden, add character and interest to a display garden, or if you want to grow for a special event.

They are relatively easy to grow but have additional care requirements beyond other summer flowers. If you can grow tomatoes, you can successfully grow dahlias. Let’s get you started on your Dahlia growing journey with some basic information.

Where and What to Buy

Dahlias come in all the colours of the rainbow except blue, sizes range from 1-inch bloom to whopping 16-inch giants, and there are now 19 forms recognized by the American Dahlia Society. If you are looking for a uniform bloom you may look to a formal decorative or ball form, or if you are looking for more whimsy in the garden you may be drawn to a cactus or anemone form. 

While there are some highly sought-after varieties in the dahlia growing world, with thousands to choose from it is best to start with what you are naturally drawn to. I suggest you choose a few in a colour and form you like and experiment with what works well in your garden space and the current theme in your backyard.

You can source your tubers from a variety of places including big box stores, local dahlia society sales, online catalog sales, or from friends growing them.

I suggest purchasing tubers from reputable small growers instead of big box stores because you are more likely to get quality tubers free of disease AND you are supporting family-owned businesses. Tuber pre-sales start happening as early as November and go through March. When ordering tubers online keep in mind that you won’t start getting your shipments until Spring depending on what zone you’re in. (This reduces the chance of your tubers freezing in the mail) *Pro Tip: Start a list of varieties you order so you don’t duplicate your purchases.

When and Where to Plant

Just like tomatoes, dahlias are susceptible to frost, so if you are planting tomatoes in your area you can be sure that your tubers can go in the ground at the same time. Generally speaking, you want your soil temperature to be around 15°C (60°F) when planting your tubers. 

When choosing a garden location remember that your dahlias need at least 6-7 hours of direct sunlight and prefer slightly acidic soil around 6.5. Also, make sure your soil is well-draining. Dahlias like a good amount of water when growing, but will rot easily if sitting in wet soil.

How to Plant

Generally speaking, you want to plant your tubers 45-60 cm (18-24″) apart and 10-15 cm (4-6″) deep. The hole should be a couple of shovels wide as you will lay the tuber flat on the soil. If you are staking each individual plant you will want to do so at the time of planting so you don’t spear your tuber later. Most dahlias need staking to prevent them from breaking later in the season. I add a small handful of bone meal at the time of planting to encourage root growth. Do NOT water until you see green growth!

Watering and Fertilizing

You do not need to start watering your tubers until you see green growth above the ground. This means you do not need to water in your tubers at the time of planting. If watered at this stage there is a high chance the tuber will rot. Watering becomes more important as the plant grows and you head into the hotter days of summer. Dahlias drink heavily to help them push out blooms and new growth. Make sure you are watering deeply 3x/week. An easy way to do this is by using soaker hoses or drip irrigation to get the water where it is needed most (to the tubers).

The fertilizing needs of a dahlia change during it’s growth cycle. It is best to have your soil tested at the beginning of the season to help you determine what your specific fertilization needs are. In the early stage of growth when plants are about 10-12 cm (4-5″) tall you want to encourage green leafy growth with a boost of nitrogen (depending on your soil test). Later in the growth cycle, you want to encourage flower and tuber growth by adding potassium and phosphorus respectively. Dahlias are heavy feeders, but fertilizing depends on your specific soil conditions. Discontinue fertilizers a month before the first frost to allow the dahlia to focus on tuber production. 

Pests in the Garden

Make sure to be on the lookout for pests in the garden. Slugs, earwigs, spider mites, stink bugs, root weevils, and aphids are all common pests that lurk in your garden. For more information on how to control pests in your area ask a local Master Gardener.

Maintaining your Plants

Dahlias grow fast and need more care than other plants in your garden. You will want to add a stake at planting time to tie them to as they grow to prevent breakage from the wind. Start tying off plants, with garden twine, at 30-45cm (12-18″) and again at 60-90 cm (24-36″). The larger your plants get the more you will have to tie off for support. When tying the plant to the stake don’t tie it too tight, you want to support it, not strangle it.

Pinching (also called stopping) will encourage your plant to branch out and produce more blooms later in the season. You simply pinch out the middle of the plant leaving 2-4 leaf pairs when the plant is a foot tall.

Disbudding is important to do if you want to help the plant produce larger flowers and nice long stems for cutting. Each stem will produce 3 buds. You want to pinch off the two side buds allowing the larger middle one to grow. You can also pinch off the second set of buds at the lower leaf nodes to ensure longer stems.

Until Frost do us Part…

Dahlias will continue to put on a show until the first hard frost. You may notice a slight change in colours from your early season flowers because of the temperature changes, but they will still be show stoppers. 

At this point, you have a huge decision to make. You can choose to do nothing and treat your dahlias as annuals. You then need a plan to purchase new tubers in the spring for your garden the following season.

Or… you can dig your tubers up to store for the winter.

If you go this route you will find that your one tuber that you planted has now multiplied to 4 or more magically in the ground over the growing season. You will then have more plants to grow the following season or have tubers to gift to your friends who fell in love with your garden this summer. 

Look for a follow-up blog on how to dig up and store your tubers.