How much water do Dahlias need? That question will always come back with an answer determined from the condition of your soil and weather.
Dahlias and soil are like Salt and Pepper, a marriage between the two and should not be separated except during a cold spell. They can only have a thriving relationship when the moisture and fertility of the soil is to the Dahlia tuber’s liking.
Often dahlias are planted quickly and the condition of the soil is an afterthought with new gardeners. If you are having trouble growing grass in an area that is a blinking-neon-warning sign Dahlias won’t survive there either.
Let’s rotate back to moisture in the soil. Dahlias do not need very much moisture to get started growing. The more they grow and put out foliage the more moisture they will need to support the water in the leaves. When Dahlias start blooming you will want to make sure they are watered well if you are not in a region that receives quite a bit of rainfall.
Is your soil on the heavy clay side or is it silty and sandy? This will determine the amount of water Dahlias require. Clay soil holds water longer and does not drain well whereas water will run right through sandy soil and Dahlias will need to be watered more often.
Ironically what both clay and sandy soils need is compost. The miracle worker for all types of soil. It makes the clay soil loamy and free draining of water and it helps the sandy retain moisture and nutrients. You have to love the benefits of organic compost.
The number one failure when growing Dahlias is too much moisture/water and the tuber suffocates and rots. It may have put up a leaf stem and then it wilts. Add cold temperatures and too much moisture, the Dahlia might not even put up a shoot, it will miserably fail, too. Warm weather with the soil temperature around 50 plus and not too much moisture to begin with when you first plant out your Dahlias is the best practice.
If you are starting your dahlia tubers in pots it’s best to do it in a hoophouse or an area without much rain. One year I started a number of tubers in pots on the driveway and it rained every day for a week. I lost quite a few from rot. They were just getting started with no to little feeder roots to take in moisture. It was like someone standing over them with a hose for 7 days and they did not like it. I also had a soilless mixture that had more peat than I would have liked that I used. Peat just soaks up water like crazy and holds onto it like it’s the most precious thing, and it is. None of us can live without water but too much of it is not the best thing either.
The best advice I can give is to judge by looking at your soil, your plants, and your rainfall. There is no one Rx like water twice a week, etc. You will be the garden doctor and determine when to water according to your rainfall, weather and the loaminess of your soil.
Happy Dahlia growing.