My latest Instagram posts have spurred many questions on how to effectively support dahlias. I can honestly say that I have tried many methods and as much as I hate my current method, it is the best way, IMO.
So to keep it as simplistic as possible...
How are you going to water? If you think - drip irrigation - then it needs placed now.
Dahlias are a long game project, best to be proactive than reactive so the netting support needs implemented early on, before your plants get too big. Or else the netting support does not help at the base and the plant doesn't grow through the 6" squares as it should. You'll have to manually work the stems through it and that takes oodles of time. I have done it, take my advice!
The netting comes in a few widths and varying lengths. Since my rows are narrow I use 3' wide netting and purchase in 'bulk'. The roll has over 3,000' on it. I should mention the netting is not a cheap option. And it requires quite a few feet depending on your operation. I plant in long straight rows to maximize my space, making it easier to run irrigation and support the plants with netting. If you utilize another lay out, you may have to think outside the box as to how this support system could work for you. The netting doesn't need to be fastened to the posts. It just needs to be stretched tight over the posts and be taunt. You do not want any give in the netting. Again, it's there to be a grid that the plants grow through and will keep them upright. There is more stretch to the netting then you'd think, so test it's limitations and if you break a section, have a few zip-ties on hand to repair.
I place 2 levels of netting. I don't measure but I would say the first layer is 15-20" from the ground and the second layer is 34-40" from the ground. Do not stress out too much on how it 'looks'. As long as the netting is tight in a month and a half you will never notice it because you're plants will swallow it up! Occasionally you may have to thread through a side shoot that didn't grow through the grid or add in a post because the plants got too tall the wind forced them over, but you won't have dahlias flopped over on the ground with stems growing at a 90 degree angle.
Supplies can be purchased anywhere. I encourage you to find a produce/orchard/greenhouse/farm supply store near you. Do a google search for your area. For Pennsylvania, there are two known suppliers: Martin's Produce Supply and Nolt's Greenhouse Supply.