Fixing a broken SFG box?

The 4x8' box that my SFG was a sandbox built by the previous owners of our house. I emptied it, moved and it, and filled it w/Mel's mix last year. I don't know the type of wood or if it was chemically/pressure treated.

Something is eating the wood - it's very brittle and some parts of the board have piles of sawdust around it. I am not sure if the box will survive another Wisconsin winter.

Any suggestions on how to fix this? Should I remove the deteriorating board and add another? I have some perennials in there that I don't want to disturb but will if necessary.

Would adding a second thickness of wood work?

Peculiar situation. Two options I can dream up. Make two separate L shaped side replacements same size as or a bit bigger than the old ones. Set them close to the existing frame then loosen the MM inside the existing walls with a flat spade etc, try to compress it inward slightly to firm it. Gently pry the two corners apart in the old box that are at the open ends of your new pieces. If a little MM caves in just brush it aside till done. Slide the old pieces out and the new ones in closer. Use a crank up binder strap or two all the way around the new box to tighten it up and secure with new screws. The other way is to just build new sides over the old ones which would be my last choice plan since you are not sure about treated wood or rot or ants or termites destroying the old wood. If you are careful and have a bit of luck few roots should be disturbed. This early in the season there is plenty of time for roots to mend themselves.

I assume the box is made with plain old HT lumber from the local big box DIY store? The easiest thing to do would probably be to remove the affected board and replace it with a new board. I've found that southern yellow pine(our form of "white wood" down here) can last about 3 years without being treated. Mel's mix should behave rather well with the side removed since it's been compressing over time. This should also mean that any plants close to that piece of wood shouldn't get disturbed much. If you're worried about the perennials, then wait until they go dormant during the winter to do the repair.

Remove the old board, and slap in a new board. I have a feeling that replacing the board will not remove the infestation of wood eating insect you may have(I assume it's insects since you have saw dust visible). They shouldn't hurt the plants much, but you will be replacing that board often. Good thing wood is cheap!

Another, more expensive and work intensive route, would be to make a new box from cedar or cyprus fence boards. I use cedar fence boards all the time, and even though they are thin, Mel's Mix is hardly heavy enough at 6" to warp the boards if you make 4'x4' boxes(4'x8' may work, but I've never tried it. I'd use the left over fence board pieces to reinforce the joint at the 4' mark on the long sides). At ~$2 per board, that's $16 dollars for the wood and ~$30 for 1 1/4" decking screws. You'll have a lot of screws left over though, so my suggestion to get your money's worth out of the screws is to make more boxes! Just make sure you are buying real cedar and not some pressure treated pine cedar look-alike. Cedar fence boards will smell like cedar. If you can't find cedar boards, then cyprus or even redwood fence boards might be available. Cedar, redwood and cyprus should all do fine, but in my experience, cedar works. Plain old wood gets eaten by termites down here, cedar does not(pretty sure cyprus is resistant too).

I agree with everyone else. If you think the bugs may stick around and infest again, and you have the funding, consider composite wood

We would like to build more boxes and have been looking into the composite 'wood.' It's sold it bundles and it is more than we need and what we would like to pay.

Thankfully, the board that is breaking down is the short end of the rectangular box. We need a simple solution, so we'll have the wood cut at Home Depot (we don't have the tools) and replace it with cedar.