Answer: There is a growth inhibiting chemical in sunflower seed hulls, but this simply stops other seeds from sprouting too close to them. The chemical washes off in rainwater and isn't strong enough to affect plants growing several inches away. You can plant sunflowers within a foot of other plants. The spacing isn't to provide a buffer zone between plants because of growth inhibiting chemicals, but to give the sunflower ample room to grow and fully develop its large seed head.
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