After growing dahlia's commercially as cut flowers over the last 5 years in the Pacific Nortwest, I've picked up a few useful practices, learned from some terrible mistakes and been taught a few good tips from friends and neighbors that should be of some help to novice and seasoned gardeners alike.
All this advice is based mostly on observations with some horticultural knowledge too. As always with gardening, different locations and climates may see different results and issues.
A potato is a tuber and garlic is a bulb.
A tuber is similar to a bulb, but generally has a thinner skin and is less suited to surviving cold and wet conditions. It stores energy made by the plant over the growing season, so that it can sprout back to life when the next growing season begins.
In most areas of the U.S. dahlias are not suited to being left in the ground over winter. Here in the North West the winters are wet and dahlia tubers can rot, elsewhere it gets too cold and the dahlia tubers are killed. In areas with mild, dry winters, dahlia tubers can be left in the ground all year... however you may want to dig them up and divide them from time to time.
Just like potatoes, dahlia tubers require an 'eye' to sprout a new plant. Tubers with no eyes may develop roots, but no plant will emerge.
Don't Rush Things
When you're planting dahlia tubers, wait for your frost date to pass, the soil to warm a little and check the forecast to avoid a long stretch of heavy rain at planting time. Spring showers are unavoidable in most areas and so long as you have planted in fairly well draining soil, they'll be just fine. If your garden gets really soggy during rainy periods, a slightly raised bed will benefit your dahlias... doesn't have to be a swanky wooden box number, just a raised mound of soil can work wonders.
You can sprout tubers early in pots, by a window, in a greenhouse and then transplant them into the garden and be the first in the neighborhood with dahlias, but in my experience transplanted dahlias are slightly more likely to fall over and sometimes wilt.
August is the great equalizer and no matter how or when you started them, by the end of the month all the plants tend to be at the same stage, pumping out blooms and buzzing with happy bees.
Dahlia tubers shouldn't be watered. If there's been no rain all spring and the soil is bone dry, then a little spritz or two won't hurt, but generally there will be enough moisture in the tuber to get things going. After the tuber has sprouted, take it easy with the watering as the dahlia is only just starting to send out roots and is still getting most of its energy and water from the tuber. If you give it too much water at this point, the tuber will rot and the little dahlia will flop over. Sometimes they have enough roots to bounce back to life, but that whole situation is best avoided by watering sparingly, if at all.
When a dahlia plant is a few weeks old with a couple sets of leaves, it's safe to assume that it has set out some roots and isn't relying on the original tuber for life. You can give it a little more water, but don't water so much that the soil is soaking wet, just dark, moist and crumbly. The cool thing about dahlias is that they can take in water through their leaves, so a watering can or blast from a hose with a shower nozzle is perfect.
A good way to gauge if your dahlia plants are getting enough water is to check on them at night. You'll hopefully see little beads of water on the leaf edges. This isn't dew, but sugar water pushed out of the plant because there is plenty water in the soil (the process is called guttation). In the morning the water on the leaves evaporates leaving behind white sugar crystals on the leaf tips that insects like to eat.
As the dahlia plants increase in size and start to flower, increase your watering, but again – no soggy soil and check the leaves are wet overnight and you'll be somewhere in the sweet spot.
It's not required, but totally normal to jump around and get excited when the first little dahlia babies pop their heads out of the soil. Be sure and take lots of pictures and post them to your favorite social media sites... this becomes a theme when growing dahlias.
"OMG MY DAHLIAS ARE DYING!"
Me, circa 2011
Dahlias wilt from time to time for various reasons. The trick to dealing with a wilting dahlia is to not panic and assess the situation.
If you walk into your garden on a hot summer afternoon and ALL your dahlias are wilting – then good news. It's probably just heat wilt and a quick spritz with a sprinkler or a hose will cool them down and bring them back. Or you can just leave them and they'll bounce back when the heat of the day passes. The only other reason I can think of where all your dahlias would wilt at once would be if someone nearby had got a little over zealous with some herbicide – herbicide is no Bueno guys.
If one single dahlia is wilting and the others are fine, then we have a mystery on our hands. First thing is to look around for evidence of damage to the plant. If the stem is damaged or broken, cut it off below the damaged area and wait to see if the plant bounces back (it should). Sometimes they get knocked by pets or wildlife which is enough to set off some wilting.
If the plant is undamaged, the problem may be underground. Dahlias don't like root disturbances and little rodents can cause problems by burrowing too close or nibbling on tubers. Also just weeding near a dahlia and nicking a root with a tool can cause a dahlia to react in wilting.
So here's what I do... I just ignore them and 7 times out of 10 they'll work it out. I've seen a dahlia flop over and look practically dead – 2 weeks later it's fine, as if nothing happened. At best I might mulch them or build some extra soil up around them. I'm a stay home dad of 3 and I've learned that wilting dahlias are just like huffing 3 year olds.
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