Sunflower Problems with Spruce?

Has anyone heard of sunflowers having problems coexisting with spruce trees?
I have a patch that I've planted with sunflowers (Mammoth) for 8 years or so. It is bordered by a row of forsythias and punctuated at the ends/corners by 3 spruce trees (Blue, Norway and [mumble], all former live Christmas trees).
The sunflowers used to do great. Now they get to maybe 15 inches tall and just stall.
I was wondering whether there is a known interaction issue. The trees are getting large (by Christmas tree standards), and this could just be a straight competition problem.
I ask because I need to figure out a new location for next year. One thought is on the other side of the forsythia line, which would not be as directly in competition, but might still be in range of any botanical warfare.
The volunteer sunflowers elsewhere in the yard do reasonably well, at least until the deer find them.
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Drew Lawson | I told them we had learned to change
| our swordblades into plows.
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     snipped-for-privacy@furrfu.invalid (Drew Lawson) writes:

Oops, meant that to go to rec.gardens.
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