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Yes, cover your sandbox. An uncovered box is an open invitation to cats, raccoons and insects, and rain turns the sand underneath into a damp bed for mold and bacteria. A cover is the single thing that keeps sandbox sand usable between sessions. Which cover you need comes down to two questions: how much wind you get, and where the water goes when it rains.
Those two questions rule out more options than you would think, so it is worth answering them before you buy anything.
How dirty does an uncovered sandbox get
Dirtier than most people expect. Slate reported on research finding that public sandboxes, left open, carried more germs on average than the door handle of a public restroom, along with a risk of parasitic worms from animal droppings. That is a public sandbox in a park, not your backyard, but the mechanism is identical: an open box of soft, diggable material sitting at ground level looks like a toilet to a cat.
Add rain, spilled drinks and a few weeks of sun, and damp sand starts growing things. Kids then put their hands in it, and often their mouths. None of this is exotic, and all of it is solved by a lid.
What to judge a cover on
Three things separate a cover that works from one that spends half the summer in your neighbor’s yard.
The first is wind. A cover has to stay attached in a gust. Drawstrings and bungee cords are enough in a sheltered yard. In an exposed one you want weight, stakes, or a lid that is physically part of the sandbox.
The second is water. There are only two right answers here. Either the cover is genuinely waterproof and pitched or pulled tight so water sheets off the edges, or it lets water through and the box drains from the bottom. The wrong answer is a cover that pools water in the middle, which is what a slack tarp does, and a sagging puddle will eventually find a seam.
The third is whether you will actually use it. A cover that takes five minutes to fit gets left off, and that is the quiet reason built-in lids beat separate covers for a lot of families.
Three covers worth using
A fitted drawstring cover
The default option, and the cheapest thing that properly works. These are polyester with a waterproof PVC coating, sized to standard square and round sandboxes, and the drawstring pulls them tight under the rim so the sand stays dry and animals cannot get under the edge.
They will not stop every insect, but they stop animals, and keeping the sand dry is most of the mold and bacteria problem solved. The limitations are honest ones: the fabric is not as tough as a rigid lid, and in a properly windy yard a drawstring alone may not hold. Measure your box before ordering, since the fit is the whole point.
A sandbox with the cover built in
Ready-made hard covers for existing sandboxes are surprisingly hard to find. If wind is your main problem and you are buying a sandbox anyway, buying one with the lid built into it solves the problem at the source.
The good ones do it twice over. The cedar box below has bench seats that fold flat to close the box, plus a retractable canopy above in oxford cloth and PVC that shades the kids while they play and drops down over the closed box afterward. Nothing to store, nothing to blow away, and cedar is naturally insect-resistant as well. The canopy comes off for washing.
The obvious downside is cost, since you are buying a sandbox rather than a cover, and it needs assembling. I have not used this one myself. It is here because the fold-down-seat plus canopy design is the only common answer to the wind problem that does not involve stakes and bricks.
A solid wood lid
For an existing box in a windy spot, a plywood or board lid is usually the practical fix. Wood blocks animals, insects and water, it holds itself down, and it looks better than a tarp. Bricks or stones at the corners are enough to keep it seated in most weather.
Two things to get right. Use untreated wood, or wood treated with something you are comfortable having next to children, and accept that any wood lid left outdoors will degrade and may attract termites. Plan on replacing it every few years rather than treating it as permanent.
Thin, flimsy sheets are the failure mode here: they warp, they flex, and they blow over. If you are going to build one, build it properly.
What about a plain tarp
A hardware-store tarp is the cheapest cover there is, and it is waterproof, which puts it ahead of mesh. It works if you are willing to do the work: pull it genuinely tight, and stake it or weight it at every edge rather than tying two corners and hoping.
Left slack, it collects a pond in the middle, and it is not pretty. If that trade is fine with you, an ordinary poly tarp and a handful of ground stakes will cover a sandbox for very little money.
Mesh and shade cloth are the option I would skip. Mesh does keep animals and most insects out, and it never pools water, but only because the rain goes straight through it into the sand. If your worry is mold, bacteria and mosquitoes, mesh does not address it.
Cover it, then keep the sand clean
A cover slows contamination down; it does not stop it. Sand still needs turning over and sanitizing periodically, and a cover does nothing about insects already living in the box, which is a separate job.
The one time a cover does most of the work on its own is over the winter. If the box is going to sit unused for months, a well-fitted waterproof cover is the main thing standing between you and a spring full of black, wet sand. There is a proper routine for closing a sandbox down for the season.
If you are shopping for a box now rather than a cover, look at what it comes with before you look at anything else, and if the fill itself is the problem you are trying to solve, there are alternatives to sand that stay cleaner on their own.
