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Growing dahlias requires some thought and care but is quite straightforward...and extremely gratifying!
It all begins with a dahlia tuber which often resembles a small sweet potato- with a body, neck, and bit of the old stalk where an eye has formed (or soon will).
WHERE Can I Plant Dahlias?
Ideally, dahlia tubers should be planted in well-drained soil which is rich in organic matter in a location receiving ~8 hours (or more) of sunshine per day.
Most gardens containing soil that has been amended over time will be more than adequate but it is important that the area is not heavily shaded. Dahlias receiving inadequate sun will not thrive...and may not even bloom.
Dahlias also prefer a slightly acidic soil but in our experience they can tolerate mildly alkaline soil as well. (Dahlias apparently prefer a soil pH of 6.3-6.8 but ours is 7.0-7.4.
For space-challenged gardeners, growing dahlias in containers may be the only option. But while we've tried this on a small scale it has never yielded ideal results for us.
WHEN Can I Plant My Tubers?
Dahlias are frost-sensitive plants and their tubers are prone to damage or loss if subjected to freezing temperatures. Although we are mindful of the frost-free date in our planting zone, we are never in a hurry to get our tubers in the ground. They only grow well in soil that has begun to warm.
As a rule-of-thumb, we plant tubers after the threat of freezing temperatures has passed and the soil is about 60°F or warmer.
HOW Do I Plant Tubers?
Tubers are best planted about 4" beneath the soil surface laying horizontally (on their side) with the eye facing up toward the surface. When planting multiple tubers we space them out 18-24". Although we've grown them as close as 14" to each other, in our experience they do noticeably better when given a bit more space - and the extra flowers they yield when they're happy more than compensates you for the extra space!
In our case, we plant tubers in 3' wide beds that are 100' long - and we plant in two rows in each of these beds. To reduce or eliminate weeds we do not use landscape fabric for our dahlias like we do for sunflowers or zinnias. And we never use weed-killer sprays which can damage or kill our tender plants. Instead, we hand-weed until our dahlias are about 12" tall then we either use chipped leaves (stored from the previous autumn) or fresh grass clippings as a natural mulch.
You may notice that different growers use different growing techniques - but dahlias that are crowded or which compete with grass and weeds for water and nutrients tend to provide fewer flowers and much more prone to disease than those which have adequate space.
And while we do supplement the soil with phosphorous- and potassium-rich fertilizer at the time we plant, we do not water at this time. Healthy tubers normally contain more-than-adequate moisture to get started and send a shoot to the surface. Watering newly-planted tubers risks them rotting due to excess moisture.