Cold-season "poop beasts"

We have an idea for a greywater mulch pit design that, with lots of tricks, could stay warm enough to function outdoors in Montana in winter, but we need a plant . . . a special plant . . . something that will grow in, say, Seattle, in the winter, and not go dormant--so it can still take up nutrients in winter. And it is what Paul calls a "poop beast"--something that can tolerate high fertility/high nitrogen (like from gross kitchen sink water). Preferably NOT a food plant, and doesn't get too tall. And we can't think of anything. Do you have any ideas?

Apparently, willow roots will continue to grow all winter:

The best time to plant willow cuttings is in the fall or very early spring — when we call the tree dormant. Actually, only the leaves are dormant. The roots continue to grow all winter from stored energy, and when the buds burst in the spring, the new leaves will have a healthy system of roots to provide them with moisture and minerals.

from this article:

https://www.motherearthnews.com/nature-and-environment/trees-for-soil-erosion-zmaz86mazgoe

apparently pussy willows are some of the earliest to come out of dormancy in spring