In theory, a “good” compost has reached a high enough temperature to kill weed seeds, say around 160F. However, most of us who make our own compost don’t have that perfect balance through the entire pile and some seeds survive.

My solution is to put down compost now, in the fall season (northern hemisphere), about 3 inches deep when I have that much, then top with about 6 inches of straw.

The straw tends to provide enough of a barrier for weed seeds to germinate. On rare occasions, the straw itself has seeds in it, but even then, they seldom germinate in my garden because the seeds aren’t in the soil.

Just to recap:

1. any viable seeds in the “old soil” are smothered by the compost
2. few viable seeds in the compost sprout because they’re covered in a deep layer of mulch
3. few viable seeds from the straw sprout because most of them are suspended in the straw where conditions aren’t right for germination

Because I said “few” a couple of times above, I should note that I do get a few weeds that pop up in the mulch the following spring/summer. But those really are few, and they’re extremely easy to pull because they’re rooted in that fluffy layer of compost.

And I also get the benefits of a deep, compostable mulch:

1. It holds in moisture, because:
   1. it provides a “blanket” over the soil that reduces evaporation
   2. it keeps the soil cooler, also reducing evaporation
   3. I buy my hay in the spring, and it sits in bales all summer, aging a bit. When I put it down in the fall, it has lost its waxy coating and it pretty absorbent. So the straw itself holds moisture in its fibers.
2. It provides insulation for the soil, keeping plant roots (and soil life) warmer in winter and cooler in summer
3. It breaks down fairly rapidly, even in my dry climate, which provides food for soil life, which in turn produce nutrients for my garden
4. It separates my vegetable plants from the soil, reducing the exposure to pests and particularly to diseases. This is very beneficial for vegetables that sit on the ground, like winter squash.

The one downside is that slugs just love the shelter (and probably food) that the straw provides. I have to put down organic slug bait[1] when I have fresh seedlings in the ground. Once plants are established, I don’t really have a slug problem, as they prefer partly-decomposed plants to mature leaves. As I pick off leaves from plants for various reasons, I just drop them on the mulch, and this provides a bit of a decoy (or offering) for the slugs and other hungry critters. And guess what? They poop nutrients right into my garden!

Footnotes