Dahlias are originally native to Mexico, so they should be able to handle any heat you'd get in Oregon. We've had 3 back to back hot summers from 2010 through 2012 in Delaware, where temps were regularly in the 90's, even up to 100 degrees,, with high humidity, and as long as the plants were kept watered it didn't seem to bother them much, however last year I was stuck growing mine in pots of about the same size as yours, maybe 5-6 gallons, and they DID NOT do very well in pots. These were all full sized 4 foot size dahlias, and I had about 18 of them in pots. I think it's a combination of the fact that pots dry out very fast, and when the soil completely dries out its difficult to moisten it well unless you get lucky and get a full days rain in mid summer. Also, their tubers are more confined and their roots can't spread like in the ground. But in Oregon I'd imagine you get more rain than we do in most years(except this year, as I'd bet we have you beat this year!) so if you are getting excessive rain like us, that could be the problem, as we've had mostly cooler weather, with just a few days of heat, and we've gotten 7-8 inches of rain in just the last 30 days along with the generally cool weather, and that's caused some of mine to rot and to grow slow.

For me, I will only grow small dahlia plants in pots from now on, and I will only grow larger dahlias in the ground.

Where are you at near Seattle? I am on Vashon Island. I could probably wave at you from the ferry boat!

If that one is virused why grow it? Why not start with something new and healthy? The virused ones just don't produce that well for me...I pulled 60 out of my 200 last fall. Not sure all needed pulling but they were weak plants and I wanted just strong healthy ones. So far, my plants are looking good this spring! I bought a whole lot more and bought from good growers.

I hope you are watering your dahlia more now? As long as the water drains well from the pot I would not worry about over-watering it.

I am actually NE of Seattle near Woodinville. Need to make it to Vashon soon.

Now that my back is better. I am back to a more normal watering schedule. Every 2-3 days depending on our sun around here.

They drain quite well. I put a layer of rocks and drilled about 5 holes per pot. The pots I have them in are probably about 25 gallon pots. They are quite heavy :) 18 inch wide square and about 20 inches tall. They should have plenty of room to not get root bound.

It is my first year with Dahlias. I was not sure if it had a virus or not. :( It is looking much better now that I watered it. The molted colors are still there though.

I would consider growing them in the ground, but don't have a good space in the yard. Also, lots of deer in the yard and I am sure they would love for me to give them more snacks. ;)

On a side note... I am a little bummed. I had a nice little small dahlia and I went to top the plant and accidentally cut off one of the side buds too... so it will not produce as many as I like.

Can I top the side bud and get two more from the side bud and fix my issue?

"Can I top the side bud and get two more from the side bud and fix my issue?"

Without seeing the plant, its hard to speak about what one would do, but it sounds like you have the fundamentals down already. This is early enough in the season that I wouldn't worry about 'getting it right,' and just go with your hunch. Dahlias are amazingly resilient and determined growers once they get out of the gate. Even if you 'do it wrong,' it won't take the plant long to recover and go to town doing what dahlias do best- bloom!

Cheers!
CC