Ask Mr. Smarty Plants

Sunflower seed may be allelopathic in some situations, and certainly sunflowers will freely sprout everywhere, often with the help of birds, who are no doubt delighted that you are contributing to their diet. You should never use the seeds as mulch without first composting them for at least a month. The heat of the compost will help to leach out the toxins in the sunflower seeds. If you are going to mulch with stalks and leaves, removing the flowers and seeds, you would probably not need to compost first. Sunflowers grow so fast and so tall that they often will shade out weeds at their feet and/or kill them with the seed toxins. Most of the information I found on sunflower seeds damaging plants involved plants under bird feeders. The discarded seeds and hulls can build up to quite a depth if you are a devoted feeder of wildlife, and will certainly smother just about anything that tries to come up. In your case, you might just let the birds (finches are especially fond of sunflower seeds) enjoy the seeds still on the plants, then clean up the leftover debris and discard it NOT as mulch, at least until it has been composted.