Sunflowers

I have planted sunflowers behind my tomato plants this year. These are the kind of sunflowers that grow large - really large.
In the back tomato beds the sunflowers are towering over the tomatoes and are over the fence already.

But in the front bed, they are much smaller. They were planted on the same day and with the same soil preparation and get the same sun - in fact the front bed probably gets a bit more sun.

We are thinking that the roots from the maple tree that invade this bed are taking nutrients from the soil. My husband thinks we either need to cut down the tree (goodbye shade) or dig a three foot deep barrier filled with cement around the Potager to keep out the roots (a bit excessive). He thinks the roots will spread to the other boxes if we do nothing. I'm going to try side dressing with some compost and seeing if that helps.
Any helpful thoughts are welcome.