Solanum Rantonnetii (aka Paraguay Nightshade, Blue Potato Bush) seems to require constantly moist soil according to the literature. And it can be pruned to keep it in shape. Could drop leaves in a cold snap - do you get cold snaps where you are? All parts are toxic if ingested.

I don't know of any mint that can't be pruned down to the ground, especially at the end of the growing season, and not have it bounce back.

If you're talking about the sort of mint you add to juleps or make mint sauce with for lamb: right at ground level you'll see new young leaves (though, by now, they're probably taller...) You can take back the old dead stems to the ground AND get some compost (you can buy it in - oh heck - whatever 20 or 40litre bags translate to for you. I don't know whether you buy it by weight or volume.)

Without smothering the plant, you can sprinkle compost over the plant and then water it with a gentle spray of water to get in down to the roots. Mint has a lot of surface roots, usually, and will respond to being fed this way.

If you're growing mint (Mentha) then it sounds that at least part of your garden gets seasonal ponding - regardless of what happens in summer.

On the other hand: if you're growing an Aussie mint bush (Prostanthera)- which has little leaves and dainty mauve or white flowers usually - cut back with care. Stay out of the old wood (a different colour from the newer wood). You can mulch it with compost or bark, but take care with any fertiliser - it has to be really LOW in phosphorus. High nitrogen, moderate potassium. The Osmocote brew I have for 'native' plants is N17-P1.6 -K8.7.

Depending on what you want to grow - feeding the soil with compost is probably the most cost effective thing you can do for most plants. Just know it can take a few weeks to show the effects (sometimes longer). Compost or humus - plants that have rotted so far you can't tell what they were, and it looks like lovely soft, dark dirt - helps the soil to hold moisture, makes it easy for roots to forage out for supplies, makes it easy for worms to work, and the soil to warm up in spring. Without it, you can add fertiliser by the ton every year and see less and less return from it. With it - you spend so much less on fertiliser and water and see far more result.