You sure its the N-surplus? How much sun do they get? Are weeds or other flowers very lush?
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Good thing about N: quite often it is added as NO 3, Nitrate, which gets washed out by the rain and transported downwards very easily. So either just wait till next season and let the water do its job. Or to be on the safe side, plant/sow sunflowers that are heavy feeders, or plant cabbage, next season I reckon. And remove all plant parts in autumn, in order to remove all the N-stuff. Otherwise there is no quick fix aginst N that I can think of.

Any chance of a pick to get a better idea?

Well, good luck, bye, Lin

You keep asking the same question.

I have never heard of a way to lower nitrogen in the soil, other than time, and water. It does wash away. Adding other things will change the balance, but it wont lower nitrogen.

You said that last year your plants did really well. Did you do a lot of fertilizing with a heavy nitrogen fert this year?

You may need to just chalk this up to a learning experience. At this point in the season you shouldnt be doing any fertilizing at all. Next year just add some bone meal when you plant the tubers and stay away from the high nitrogen stuff.

Just because the leaves are dark green does not necessarily mean that the plants have too much nitrogen. Do they have overly long stems as this is symptom of too much nitrogen?
Perhaps the pH is wrong as dahlias do well at about 6.0 to 6.7 pH. When the pH is off the plants cannot take up fertilizer even when it is present and do not grow well. .

Yes, teddahlia, I do have long stems and huge leaves. I will water ot the raised beds at the end of the season. Pat

I would want to save the nitrogen for next year instead washing it away. In most areas it disappears over the winter. What was your source of the nitrogen?

OK, I will do that. In the Sprig I will try and find a 0-10-10 fertilizer. They are growing leaves and laterals like mad but we don't have the time. Pat