Hi Jack, thanks for using Ask Extension.  

Managing unused growing areas can be challenging.  There are many options to consider.  It is best not to leave it to the weeds, since that would add to the weed seed bank.   In considering cover crops you need to consider how you will terminate the crop.  Maintenance and timing and are also factors. 

Clover and ryegrass both grow vigorously and can be difficult to remove once established. Clover spreads by stolons and often survives tillage. Annual ryegrass can reseed heavily if it matures, and cereal rye can be hard to terminate and may delay vegetable planting due to allelopathy. These traits make them poor choices for community gardens where different people use the plots each year.

Oats can be a better option.  They are easy to establish, suppress weeds well, and winter‑kill naturally, so they don’t become a problem the following spring. They leave a soft mulch that’s easy to plant into.

An oats and field peas combination could also be considered. This mix adds some nitrogen and also winter‑kills. Peas can volunteer the next spring if they were allowed to form pods, but these seedlings are easy to remove.

If maintenance will be limited, mowing the weeds and covering the soil with cardboard and straw or wood chips is a simple way to keep weeds down and protect the soil without creating termination issues next year.

For unused vegetable plots, oats are probably the safest and easiest cover crop. An oats‑and‑peas mix also works if peas are mowed before they set seed. Clover and ryegrass are not recommended because they can be difficult to remove and may interfere with next year’s vegetable planting. 

Check out the following links:

Choosing+Cover+Crops+-+Urban+Areas.pdf

Using cover crops in the home garden - Agriculture and the Environment

I hope this is helpful.