On Tue, 30 Jul 2013 14:35:57 +0000 (UTC), Drew Lawson wrote:
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 see about four possibilites:
1) the spruce are hogging the soil moisture, and/or the sunflowers
aren't getting enough applied water.
2) the spruce are shading the sunflowers
3) you've got a pathogen or perhaps something like a nematode,
stunting the plants in that area.
4) you've got a nutrient deficiency (sunflowers like fertilizer)

A patch planted elsewhere would probably be a good idea.
If you want to explore the other hypotheses, consider
digging out a few big pots-worth of soil next spring,
a couple from the current spot and a couple from the new
spot. Dig in a heavy amount of well cured compost in half
the current spot. That will fix most of the
soil pathogen/soil pest issues. If compost side does
better than untreated side, that's probably the problem.
If both stall out again, try watering at an inch a week.
If that doesn't do anything after a week, try some general
purpose fertilizer.


You can use the potted sunflowers in soil and
shade to compare the same soil to each other with shade and sun.

This is hardly a statistically reliable experiment, just something to
help get a handle on what might be going on.

It's also possible that you can get somewhat of a crop this summer
if you replant now.

Kay