red dahlias, from seed

I'm embarrassed by the weeds these dahlias are competing with in my garden, but here, they look positively "kept." And I love the fact that they're small enough to use in windowsill vases. I've got two patches of them in the garden--both grown from seed--and one patch has plants about a foot tall in it, the other plants about two feet tall. Both have about the same number of harvestable flowers, but obviously I can cut flowers with longer stems from the second patch. Seems I should be able to account for why plants in the second patch are so much taller,  but I can't. It could be that I added more compost to the soil there, but plants there might also be taller just because they're jammed in with other tall plants requiring them to stretch for the sun. What a puzzle this gardening business is!