How to Grow Dahlias
When to plant dahlias
Plant dahlias when the ground temperature is at least 15°C and all danger of frost has passed. Generally, when the ground is warm enough to plant tomatoes, you can plant dahlias. If you plant when the soil is cold and wet, the tubers will rot before they can start growing.
Where to plant dahlias
Plant dahlias where they’ll have full sun, at least 8 hours a day. They’ll need a spot that has good drainage. Don’t plant anywhere water pools. Dahlias won’t tolerate soggy soil and the tubers will rot if the soil doesn’t drain freely. Make sure your watering hose can reach your dahlias. They’ll need a lot of water as they grow larger, especially on hot summer days.
How to plant dahlia tubers directly in the garden
Prepare the soil
It’s very important to prepare the soil so your plants are healthy and produce lush flowers. Dahlias need a sunny spot with warm, well-drained, fertile soil. In a 12” wide circle, 8” deep, loosen the soil and mix in:
- about 3” of compost or well rotted manure. Don’t buy soil that has been treated with weed killer or fertilizer, it will burn young dahlias and they may not survive.
- a light dusting of bone meal if you have some on hand. If you have nosy pets or animals around, you can skip this step because they’ll dig this area.
- lime (increases PH) or sulphur (decreases PH) if you know your soil PH, as dahlias prefer soil with a PH of 6.5 to 7.0.
Plant the dahlia tuber 4” deep, lying completely horizontal, with the eye or sprout facing up. It sounds odd, but do not water until you see the shoots appear. The moisture in the soil should be enough. It’s best to insert a support stake (wood, bamboo, rebar) at this point, so that you don’t accidentally spear the tuber later. Don’t forget to add a label! Space your dahlias 15” apart so they have enough room to flourish.
How to start dahlia tubers early in pots
We prefer to start our dahlias early, indoors, to get a head start on the season and have more control over the environment. You won’t have to worry about frost, heavy rains or pesky critters. Pot up your tubers about 6 weeks before the last frost date in your area. They need a warm (at least 22°C ), sunny spot so that the tubers can wake from their dormancy. A heating mat under your dahlia pots would be ideal. If you use a heating mat, you’ll need to water more frequently so the soil does not dry out.
- You’ll need a 1 gallon pot (6”-7” wide and tall) with drainage holes. You can use a smaller pot if your tuber is small.
- Use potting mix that doesn’t have weed killer or fertilizer since it can burn tender roots. If you don’t have enough potting soil, mix with garden soil 1:1.
- Fill the pot and plant the tuber horizontally, about 2” deep. If your tuber is large, you can plant it at an angle. Just make sure the end with growing eyes is raised and pointing up. Cover with soil.
- If your potting soil is fresh from a sealed bag, it’s probably moist enough and you don’t need to water until you see sprouts. If your potting soil is dry, water lightly, making sure any extra water is able to drain freely. It can take dahlias 3-5 weeks to sprout.
- Once your plant has 4 pairs of leaves, pinch/cut off the top of growing centre to remove the top pair of leaves. It may seem counterintuitive, but pinching encourages branching and more blooms. If you don’t pinch, plants will have a thick main stem and less flowers. Water regularly as you would a house plant. Don’t overwater or let the soil dry out.
- Before planting your dahlia pots outside, you’ll need to harden them off so that they aren’t shocked by sun, wind and temperature. Over the process of a week or two, gradually expose your plants to the outdoors. Begin by putting them outside for a few hours on a cloudy day or in a shaded area. Choose a sheltered spot so they aren’t beaten by wind. A cardboard box works well for this and makes it easy to move plants around. Make sure to bring plants in before night. Repeat this process giving your plants a few more hours outside each day, always remembering to bring them indoors. Gradually increase their exposure to sun at the same time. At the end of the week, assuming weather permits, you can leave them outside overnight. After a few more days and nights completely outside, your potted dahlias should be ready to plant out.
- To plant your dahlia pots outside, follow the steps for “How to plant dahlia tubers directly in the garden”. Plant so that the dahlia tuber is about 4” deep.
How to pre-sprout dahlia tubers
Whenever you plant a tuber, it races to start growing before rot sets in or it dries out. It can take 3-5 weeks for a tuber to sprout, so if it rains a lot for a few weeks, dahlia tubers planted directly outdoors may not survive. But if your tubers are already pre-sprouted, then they’ve already started growing and have a better chance of taking off when planted. Like any plant materials, dahlia tubers are perishable and some may not sprout. Pre-sprouting is a great way to make sure you’re only planting viable tubers and not wasting any space, soil or time.
- Use a tray or shallow pot filled 2” deep with moist potting soil, vermiculite or peat moss. Again, make sure there are no weed killers or fertilizers in the mix.
- Label your tuber with a sharpie or tag. Place the tuber horizontally in the mix. Tubers don’t need to be completely covered with soil, just enough so that they don’t dry out.
- Space tubers about 1” apart. It’s ok that they’re close together because they’ll only stay in the tray until they sprout. Place the tray in a warm spot (at least 22°C ) or on a heating mat. Keep the soil moderately moist so the tubers don’t dry out and the future roots can drink.
- You’ll know the tuber is no longer dormant when you see a waxy green/red sprout. Your dahlia tuber is now ready to plant out in the garden, or in a pot if the soil is not warm enough outside. You can let tubers grow in the tray until the shoot forms a few leaves, but avoid letting it to grow so much that the roots get tangled.
How to care for dahlia plants
Now that your plants are out in the garden, check on them regularly. When the plants are young they don’t need very much water, but dahlias are vigorous growers and will need more water as they get larger and the days gets hotter. Give them a deep watering (making sure the water reaches the bottom roots of the tubers) 2-4 times a week depending on weather conditions and your soil type. Avoid overwatering as this will lead to tuber rot.
Once plants have 4 sets of leaves, pinch the top of the growing centre, removing the top pair of leaves. This will encourage more branches, more flowers and longer stems.
Dahlias can grow quite tall so they’ll need sturdy stakes (wood, bamboo, rebar) for support. As your plants grows, make sure to keep tying them to the stakes so they aren’t damaged by wind. I didn’t stake a section of 4’ tall dahlias and found that they had been completely sheared off at the base by a storm. It’s heartbreaking to lose a plant, especially a large dahlia full of flowers so stake your plants!
Slugs and snails love young plants so apply slug/snail bait when planting and throughout the season if necessary. To help prevent insects like aphids, mites, thrips and white flies, spray your plants weekly with insecticidal soap. You can easily make your own insecticidal soap spray with Dawn dish soap, water and a clean spray bottle. Mix 4 tbsp of blue Dawn with 1 gallon of water and spray your plants from top to bottom. Don’t forget the undersides of the leaves. Insecticidal soap needs to come in contact with the insects to be effective. Don’t spray when it’s very sunny, the droplets can magnify the suns rays and burn plants. Always test your spray on a small area to make sure the recipe works for your plants. To protect precious blooms from earwigs and grasshoppers, use drawstring mesh bags to cover the buds before they open. Organza jewellery/gift bags work well for this, just make sure the bag is large enough to accommodate the size of the flower when it fully blooms. Gently place the bag over the top of the bud and tie the drawstring snug so that earwigs can’t crawl in.
When to cut dahlia flowers
Congratulations you did it, you’ve grown your own dahlia flowers!!! All that toiling was worth it. Now it’s time to enjoy your beautiful flowers indoors. The best time to pick flowers is in the morning, when the plants have drank overnight and the stems are full of water. If you cut during the heat of the day, the plants are stressed, shortening their vase life. Bring a clean bucket or vase, full of room temperature water to put your stems in. Dahlia blooms don’t open much after they’ve been cut, so pick them when they’re 3/4 or almost fully open. Using sharp, clean snips or scissors, make a clean cut giving yourself at least 12” of stem. You must cut deeply into the plant to encourage more branching, more blooms and longer stems, same principle as pinching. The more flowers you cut, the more blooms you’ll get later. This is one reason for ‘dead heading’. Dead heading is just removing the spent flowers, so that the plant continues to make blooms instead of putting its energy toward seed production. Cut off at least 12” of stem when dead heading. Once you’ve finished picking your dahlia flowers, bring your bucket indoors and let it sit in a cool, sheltered area for a few hours to recover and drink. When placing your stems in their final vase, trim the ends about 1/2” with sharp snips or a knife. Making a clean cut at an angle, helps them take up water. Change the water daily to prolong the life of your flowers. Dahlia flowers last 5-7 days in a vase if your plants are healthy and the blooms were cut at the right stage. In general, I find dinnerplate sized dahlias do not last as long as the ball varieties. But no one seems to mind because the dinner plates are gorgeous and have such a presence. Just one bloom, makes such an impact!