I thought the sunflowers themselves, put out a toxin to prevent other plants around them. This toxin would also be in the seeds. I buy the cheaper striped sunflowers, and the hulls thrown about do make a bare area in spring grass growth. Seeds might also be treated with something, but I had not heard of it. Perhaps you could call a company name on the seed bag. I feed a lot of sunflowers during winter,usually a couple hundred pounds. With the raise in prices (doubled here, now $20 for 50#) I can't afford hulled or even the black oilers. My local birds like the sunflowers and suet I put out. We do have fields with unmowed weeds for birds liking seeds like thistle. With my quantity of seeds used, they make a large pile of hulls. I rake up my hulls come spring, use them in places I don't want growth of weeds or grass.

I'm with Mike on this. I would not allow the seed hulls of black oil sunflowers to collect under the feeders if I wanted plants to grow well there, from personal experience. At one point in my life, I went a bit off the deep end with a row of bird feeders hanging off the edge of my side porch. The hulls made a lovely mulch, several inches deep. I had noticed that no weeds grew under there (with the exception of a few volunteer sunflowers) but when I began to realize that the flowering shrubs were not doing well I did some research and found the same information that Mike mentions about there being toxic compounds in the shells. I raked out the beds thoroughly and the shrubs seemed to take off after that. Anecdotal perhaps, but I'm convinced.

I had read the same thing years ago, but the article specifically referred to the striped hulls.I feed all year with black oil seed ( more nutritious for birds) and have had no problems in the past several years. Now and then I'll go out and scoop up a few handfuls of hulls,and I have left a bit of space directly under the feeders so that the hulls (and droppings) don't fall into the hostas much. If there's any problem there, it's the tree roots and my refusal to water unless there's a real dry spell. My theory is that it keeps some plants small so that I can fit more in. In reality, I'm lazy about watering and we have serious water bans in my town. Oh good-I'm law-abiding, too!

I strongly agree with the recommendations to smother & WAIT. Think of this as a long-term project. Immediate results are not likely to be the best results in the long run. In my experience, weeds are one thing -- grass weeds are another beast altogether. I like the idea of using natural fiber carpet rather than plastic b/c the water can still get through. Otherwise, you can have runoff issues. If you start this in the summer, you'll have an even better chance of killing off everything. I know it's been several weeks since you started this thread, but in Dallas, we're in the middle of a run of triple digit weather, so its the perfect time for us to smother.