Tuber Store Is OPEN
We never water our freshly-planted tubers, and do not worry about watering plants until after we've see shoots emerge from the soil.
Our region in the mid-Atlantic receives good rainfall during much of our growing season. But dahlias are true water-lovers and they suffer when the soil becomes too dry.
Dahlias in dire need of hydration will show obvious signs of distress - initially in the leaves. Dahlias receiving some, but inadequate, hydration may look fine but will fail to produce many blooms.
We water our plants once or twice a week (depending on rainfall)...but when we water, we do so heavily. Water needs to get to the "root zone" where the tubers are located and this is often 5-10" beneath the surface. For someone growing a few dahlias, careful hand-watering may suffice. Larger-scale dahlia enthusiasts will probably appreciate the efficiency of a simple drip-irrigation system.
It is worth pointing out that overhead watering risks creating too moist an environment on leaves and stems which can promote disease. So, whether drip-irrigating or hand-watering, we recommend applying water near the soil surface rather than from above the plant.
Gardeners planting "a few" dahlias will probably do fine adding a low-nitrogen fertilizer at the time of planting and following-up with smaller applications of the same fertilizer at one-month intervals.
Larger-scale dahlia fanatics, however, will want to be more precise. At Rich Hollow Farm, we test our soil each year with a focus on phosphorous and potassium levels, pH and organic matter. This helps us plan ahead when determining what types of fertilizer (and in what amounts) to add the next season and how aggressively we want to add compost before year-end.
In our experience, dahlias do well with 225 lbs/acre of P and 250 lbs/acre of K. We avoid synthetic fertilizers, so we tend to mix bone meal (phosphorous-rich) and potash (potassium-rich) in amounts calculated to achieve our target concentration of those nutrients. We've never needed to worry about adding nitrogen, and over-applying nitrogen will result in a vigorous and leafy plant with few flowers.
At one-month intervals we add smaller amounts of the same P- and K-rich fertilizer but find our plants thrive without too much post-planting fertilization.
Staking and Netting Dahlias
It is hard to imagine, but we plant dahlias that routinely grow 6' tall or more (I'm looking at you Blizzard!)
But even dahlias growing taller than about 3' are susceptible to damage from wind. And many dahlia varieties produce abundant large blooms which make them prone to falling over on their own. There are a number of ways to protect your dahlia plants from toppling over and what works for us may not be ideal for you.
What we do: drive t-post stakes into the ground every 10-12' along each side of our rows and, at some point before the dahlias have grown more than about 1' tall, we use those stakes to support a horizontally-placed row of Hortonova netting (or a similar product).
In our case the support netting needs to be 3' wide (the width of our rows) and is suspended about 2' off the ground by the stakes. We cut our netting to the desired length (often 100') and we re-use the same material every year. Unrolling / installing netting over your dahlia bed is a two-person job...as is rolling it back up at the end of the growing season!
The netting stabilizes young plants and over time we can "shimmy" the netting higher on the stakes as the plants grow taller. Alternatively, we could add a second layer of netting about 4' off the ground to provide additional support.
Rather than using two layers of netting, however, we monitor our plants and simply "corral" certain sections of plants by tying brightly-colored paracord between stakes along one side of a row where necessary to prevent plants from leaning too far out of the bed - or being blown by heavy winds into the paths between rows.
Growers with just a few dahlias may find that large tomato cages will work well for individual plants.