Dahlia Do's and Don'ts

Flowers, like celebrities, can have cyclical popularity. Based on the volume of customer questions, dahlias are the smokin' hot plant right now. Never grown them? Check them out at your local garden center and prepare to be dazzled.  

A wealth of choices

Hybridizers have expanded the range of flower and plant sizes, colors and flower forms so there's a dahlia for every taste. It's pretty darned cool to watch a quarter-sized bud open into an 8" (or larger) flower later in the summer! Yep, I'm in awe of big ole' dinnerplate dahlias.

Here are a few tips for success from my experience growing dahlias over the years as well as from people who grow them by the hundreds.

Undivided dahlia clump

The smaller bedding dahlias can be grown from seed, but the large-flowered varieties are grown from tubers (potato-like storage organs). They're most often sold as undivided clumps with multiple tubers. Don't just dig a hole and plop the old clump back in the ground. Get a sharp knife and cut a tuber from the mother clump. Each tuber must have an eye, or bud, to produce a new plant. Eyes will be at the end of the neck of the tuber, coming from last year's stem. A dahlia clump might have as few as one, or as many as 3-4 eyes.

Divided tuber with eyes

Like tomatoes, nothing is gained by planting dahlias too early. They're tropical in origin and shouldn't be planted until soil temperatures are consistently above 60 degrees F. They abhor cold, wet soils.

1. Dig a hole 4-6" deep. The tuber will be placed horizontally in the bottom of the hole. Life is easier when you place the stake (you'll need a stake for these big, vigorous plants) in the hole next to the eye of the tuber. Dahlias, like vampires, resent having a stake driven through their heart, which is the tuber!
2. Some growers recommend mixing dehydrated manure into the backfill soil as the organic amendment. You get the bonus of a small amount of nitrogen, phosphorous and potassium.
3. Important: Dahlia wisdom is our spring soils generally contain enough moisture that you don't water at the time of planting. In fact, don't water until you see the first shoots breaking through the soil.
4. Dahlias are an exception when it comes to mulching. You actually want the sun to hit the cool/cold soil and warm it up. So, don't be in a hurry to mulch. Remember they respond to sun and warmth.
5. If you want a shorter, bushier plant (not a bad idea with plants that may be 6' tall by summer's end) you can pinch out the tip. As soon as the plants have more than 3 pairs of leaves, reach in gently with your fingers or a cutting tool and remove the growing point, leaving 3 pairs of leaves intact. Like most pruning this will produce a shorter plant with more shoots and potential flowering stems!
6. Big flowers equal hungry plants, right? That sure seems like it would compute. But many dahlia growers urge against lots of nitrogen. Use a granular fertilizer with a ratio of twice as much phosphorous as nitrogen. Example: 5-10-10 (nitrogen, phosphorous and potassium). Apply a month after planting, then a month later. Avoid water soluble fertilizers.
7. When we get into hot weather water deeply when needed. Remember you planted those tubers 4-6" deep. They have lots of thirsty leaves and flowers later in the summer when it gets hot. Don't use wilting as your indicator that it's time to water. Always water deeply before wilting occurs.
8. As your dahlias start budding you have a fun decision to make. Do you want masses of smaller flowers, but lots of color to be viewed from a distance? Or, do you want big, honking flowers for cutting or bragging rights (It's a guy thing, like having the first tomato on the block)? The former, do nothing. The latter, remove the buds paired on either side of the larger central bud as soon as you can handle them. The plant's energy will be directed into that remaining bud and will really increase the flower size.

Dahlias - definitely not for the fairy garden!!!             

Cactus-flowered dahlia