I also get TONS of grass in my pots, in my gardens, in my raised beds, it is literally everywhere that I grow, which is why I go through so much effort to use mulch, plastic mulch, and ground cover. Even with all of that, I spend 90% of my gardening time, pulling weeds, hoeing weeds, tilling through weeds or hitting them with the weed eater. It is a never ending battle, especially when you are organic, and especially this year with all the rain.
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I have never had a problem with Pro-Mix having weed seeds in it, of course, I am sure that some sneaks in there, there is no way they can prevent it. I leave a big container full of moistened Pro-Mix in my garage pretty much year round, and the only time I notice weeds coming up is when I mix in compost, or a couple of weeks after I fill pots and put them outside. The best I can tell, I always have the same varieties of weeds/grasses coming up, which leads me to believe the seeds are finding their way into the mix locally, if they were in there from the time of harvesting the sphagnum, I would think I would be seeing weeds that are native to Canada and not Illinois (though I am sure some are native to both places).
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So, I am not saying you didn't get a bad batch of Pro-Mix, but here is my personal theory, based solely on my experiences, it is only one possible scenario out of many:
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I think the reason you are seeing much more weeds in the Pro-Mix beds than the "other mix" is due to the superior nature of Pro-Mix. Any seeds that make it into the Pro-Mix are going to germinate, where your "other mix" may not have the right stuff to support germination. I would guess the seeds are in the other bed, but are slowed down due to it being a heavier mix of soil, a difference in water retention, or some other variable.
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I hope this helps, and I agree, companies SHOULD make an effort to take care of EVERY customer, not just the big ones.