Dahlia Growing Guide

How to Grow Dahlias

Dahlias thrive in 6 to 8 hours of direct sun, especially morning sunlight. Some growers in warmer climates will provide sunshades during heat waves, more frequent waterings are sufficient if you cannot provide shade during the hottest part of the day. Dahlias benefit from protection from wind and usually require staking, unless they are a shorter ‘border’ dahlia. They grow best in rich, well-draining soil and benefit from extra fertilizer early in the season. Amend heavy clay soil with aged manure or compost to lighten and loosen the soil texture for better drainage.

When to Plant Dahlias

Dahlias will not tolerate cold soil. Plant when the soil reaches 60ºF (15°C) and any danger of frost has passed. Most zones in Canada plant their dahlias after Victoria Day weekend, when all chance of frost has passed. Even if planted in early June, there should be enough time to reach the ‘120’ days threshold to make good tubers. Just countback from your first frost date, to make sure there is enough time.

Some gardeners start tubers indoors in containers a month ahead to get a jump on the season. If you do plan to start your tubers inside, moisten your potting soil, plant the tubers and then leave it alone until you see shoots. Frequent watering before you see green will result in your tuber rotting and no flowers!

Planting Dahlias in the Garden

To plant the tubers, start by digging a 6- to 8-inch deep hole. It also helps to mix some compost and a handful of bonemeal into the planting hole. Otherwise, do not fertilize at planting. Put the tuber into the hole with the growing points, or ‘eyes’, facing up. Cover the tuber with 2 to 3 inches of soil. As the stem sprouts, fill in with soil until it is at ground level. Do not water the tubers right after planting. This encourages rot. Wait until the sprouts have appeared above the soil, then water. Be sure to put in staking when planting, it will save you time later on, as well disturbing the roots of the dahlia less. Place 5- to 6-foot stakes around plants and tie stems to them as the plants grow.

Container Growing for Dahlias

Most experts agree that dahlias grow better in the ground than in containers. I have found the opposite, in my heavy clay soil. I frequently grow dahlias at my house in containers and tuber production is much higher than in the ground. I water very frequently and fertilize frequently and have excellent results, often the container grown dahlias bloom before the in ground dahlias. I have done grow bags, I have done Rubbermaid totes and I have done pots. I would not recommend grow bags, as they dry out too quickly but anything that isn’t porous and large enough to accommodate staking is good. You just need to have a pot that is big enough to stay upright and accommodate the very tall potential of many dahlias, particularly ‘dinnerplate’ ones.

Winter Care

Unless you are in zone 8 or higher, you will need to lift your tubers before winter and the ground freezes. In order to have viable tubers, dahlias need to be growing in soil for 120 days, which is achievable in most parts of Canada, if planted by Victoria Day. People usually wait until the first frost blackens the leaves of their dahlias to cut them down but feel to lift earlier if that works for you (as long as they have hit the 120 days needed for mature tubers). One benefit of waiting to dig up your tubers is less time that you have to store the tubers and less time for things to go wrong during storage.

Once you have cut down your dahlias, you can leave them in the ground for up to 2 weeks. This allows the tubers to harden a bit and makes them easier to store. When  you are ready to dig up your dahlias, gently lift with a pitchfork and carefully knock the soil off. Dahlia tubers will need to ‘cure’ for a couple of days, which is simply allowing the outer skin to dry off so that it doesn’t mold in storage.

Once your dahlias are cured, it is time to store them for the winter. There are a few guidelines for storing dahlias. Temperatures should be between 4 degrees Celsius and 10 degrees Celsius. They do need a certain amount of moisture in the air. They do need to be covered so that they don’t dry out. But they can’t have too much moisture or they will mold. They are very fickle creatures and it is hard to give a definite answer about what will work for your particular micro climate.

Some methods that have worked for others:

• Plastic bags in the fridge (but not the back of the fridge as they can freeze back there). I have had some success with this method, except for the ones that ended up in the back of the fridge (they froze on me!).
• Stored in vermiculite in plastic bins. This is what I do currently but unfortunately I lost my cold room due to the flood situation. They are at 17-21 degrees Celsius and the ones at 17 degrees are doing better than the ones at 21 degrees Celsius. I am holding back quite a few dahlias until I see how they fare.
• Stored in potting soil in plastic bins. I tried this and they dried out quite quickly. Up to you if you want to try it. Some people swear by it.
• Wrapped in saran wrap. My friend is trialling this method this year. I will report back next year how it goes.

Dividing Dahlias

Dividing dahlias is a skill that can only be done by either doing it or watching someone else do it. I highly recommend watching videos on it through YouTube. Some tips that I can give you is to clean off all the debris, including roots and feeder tubers. Use an old toothbrush to brush soil off gently, so you can see if there is any growing points. If they don’t have growth points, known as eyes, they most likely won’t grow. However, try to keep any tubers that you don’t think are viable, I have had a few grow that I didn’t think they would.