There are a few reasons that mulch will move:
- If it's small enough, wind can blow it.
- In an area with movement (e.g. people walking, bikes, hand-carts, golf-carts) then that traffic can push the mulch.
- Water, whether from rain or a flood from next door or sprinklers, can push the chips. If there's enough water an even worse problem is that the water will cause the chips to float which will really move them.
- The steeper the slope of the surface the more pronounced some of these problems can be.
- If there are no obstructions on the slope then it will keep moving.
- If the mulch hasn't or can't form together into a mat due to size, shape, and uniformity of the mulch material.
So, what are the solutions?
When applying mulch: apply it thick, add some moisture from something like a hose, pack it in by walking on it, terrace the slope if you can, remove flooding risk, and provide obstructions in the path, and use an ideal mixture of mulch.
- I've seen a lot of gardens where people don't want to spend money so they buy too little mulch or maybe they estimated wrong. Consider using a service like Chip Drop or just call a local arborist and request a dump of woodchips. You will likely get enough you'll be inventing uses and sharing with neighbors.
- A bit of moisture (not so much you wash it away) will help in the initial period by weighting down the material and will help it connect to pieces around it forming more of a mat structure that will stay put better than individual disconnected pieces of wood.
- Packing the mulch down, e.g. by walking on it, will also help the pieces interlock with each other into a mat, again, helping it to behave as a unit rather than individual pieces that can wash away.
- Reducing the slope, e.g. by terracing, is something you have to do in advance, in general. It also may not achieve the look or goals you have for the space. There are materials like Geocells that create a grid of small terraces and hold materials in place on a slope by creating a lot of mini-terraces. This is likely cost prohibitive for most situations, but might inspire some thoughts about solutions.
- If you have downspouts or other sources of water that will flow into the space, consider how you can either divert the water away from the space or reduce the flow. You could use a corrugated culvert to let water out in various places instead of just one spot.
- Large rocks that are anchored into the slope by digging out a platform and plants will help provide obstructions that block the mulch from sliding off.
- A mulch that is a mix of different sizes/shapes including some shredded wood will be more likely to form into a mat and stay in place. In comparison, a mulch like bark chunks that are all large have a harder time forming a mat.