Grow dahlias with this expert advice
Who would guess that a few plain-looking tubers could grow into such beautiful flowers? Dahlias bloom in a myriad of colors, patterns, shapes and sizes that will brighten your borders from midsummer to frost. And they're an excellent cut flower! Nicholas Gitts is a second-generation owner of Swan Island Dahlias, a mail-order dahlia nursery in Oregon, and has been growing these fabulous flowers for most of his life. We've asked his expert advice on how to successfully plant, grow, dig, and store dahlias.
Dahlia growing basics
Full sun, at least 8 hours a day, and well-drained soil is the best way to grow beautiful dahlias. In general, the larger-flowered dinnerplate types have fewer flowers on taller plants. Smaller varieties usually have more branching and smaller blooms. If you only have part sun, try some smaller cultivars. They don’t seem to mind a little less sun (no less than 6 hours) as much as the bigger ones do.
Tips for growing dahlias in the heat
Dahlias grow easily pretty much everywhere but can struggle where temperatures get more than 100 degrees F consistently. But if you’d still like to grow them, there are a few tips that can help in hot summer regions:
- Grow dahlias where they get morning sun and afternoon shade.
- Water so that it soaks 6 to 8 inches into the soil twice a week, instead of shallowly every day.
- A 2- to 3-inch layer of organic mulch helps conserve moisture and keeps soil temperatures even.
Fertilizing dahlias
There’s no need to fertilize a lot or use compost with dahlias. Most are usually too rich or too high in nitrogen, which produces weak stems and can burn the tuber's "eyes" (the small growth bud near the stem) at planting time. Instead, use a plant food low in nitrogen and high in potassium and phosphorus. Apply it 30 days after planting then again 3 to 4 weeks later.
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Which tubers are best?
When you buy dahlia tubers, you might notice a difference in what you get. Those from mail-order nurseries tend to be single division tubers that look kind of like a small potato, often with the cultivar name stamped on the side.
The bags from big box stores frequently have a clump of several tubers joined together at the old stem. You might think that a clump would grow a bigger plant, but that’s not so. In fact, it’s best to divide them before planting — you’ll get sturdier, healthier plants this way.
Dividing dahlia tubers
To divide a clump of dahlias, slip a sharp knife in between the tubers. As you’re cutting, twist the knife a little and the tubers will snap apart. Make sure each tuber has at least one eye, or growth bud.
Planting dahlia tubers
- Dahlias like it warm, so don’t plant the tubers outside until the soil temperature is 60 degrees F or higher or they may rot. That’s about the same time you’d plant tomatoes and peppers.
- Dig a hole 4 to 6 inches deep and lay the tuber horizontal with the eye, or growth bud, pointed up.
- Water lightly, then don’t water again until you see new growth. Dahlia tubers rot easily.
Pinching dahlias
It can take a couple of weeks to see growth from your dahlia tuber and 8 weeks or so for it start blooming. To ensure you have plenty of blooms to enjoy after that long wait pinch or snip out the growth tip when the plants have three sets of leaves.
Get earlier dahlia blooms
For earlier blooms, Nicholas suggests starting dahlias in pots indoors about 6 weeks before the last average frost date in your area.
- Plant a single tuber horizontally 2 to 3 inches deep in a nursery container of potting mix and water lightly.
- Put the pot in a place that gets at least 8 hours of light or more a day.
- When you see growth in a few weeks you can start watering regularly but let the potting mix dry out before watering again to avoid rot.
Plant potted dahlias outdoors
Just like with the tubers, the potted dahlias can go outside when the soil temperature is at least 60 degrees F. Dahlias are quite frost sensitive. To plant them in the ground, dig a hole as deep as the pot and set the plant in the hole so the potting mix is flush or slightly below the soil line. Cut the stems back to just above a leaf node about 6 inches above soil level to encourage branching.
How to stake dahlias
Dahlias have hollow stems that can snap off easily in a strong wind. So if the cultivar your growing is more than 3 feet tall, Nicholas suggesting giving it some type of support. The best time to put in a stake is at planting time. That way, you won’t damage the tuber, roots or stems later.
Different ways to stake dahlias
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Fencing, half hoop stakes, or tomato cages work well for bushy, multi-stemmed plants. And if it’s green, it will blend into the foliage so you can’t see it.
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For dahlia plants that are 4- to 5-feet, surround plants with wooden stakes. Then, loop garden twine every 18 inches to keep them upright.
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For large flowered dahlias, a single piece of wood, bamboo or even rebar works fine. Place the stake about 12 inches in the ground to it will stay upright and support the plant even in the wind. The height of the stake depends on the cultivar, but you’ll want it to be almost as tall as the dahlia at its mature height. Tie the stem to the stake with soft fabric or a plant tie as it grows.
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Save dahlia tubers to grow again next season
Dahlias are only cold-hardy in USDA zones 8 to 11, so in most areas you’ll need to dig the tubers out of the ground after a frost kills the foliage in fall. Cut the stem off to 6 inches above the soil before digging to make handling easier. See our article on How to Save Tender Bulbs has lots of helpful tips for getting your tubers through winter so you can enjoy those gorgeous blooms again next year.
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