Rock Pile

A rock pile is a loose stack of stone set in or beside a growing bed. Nothing holds it together, and that is the point: the gaps between the stones are caves, and lizards, skinks, snakes, toads and ground beetles move into them. Those animals eat slugs, insects and rodents, so the pile is a way of housing predators a garden benefits from instead of hoping they pass through. Stone holds heat longer than air does, so the pile stays warm after sunset and gives the animals somewhere to bask. The largest stones go at the bottom so the pockets form as it rises, usually from rock the digging has already turned up.

Sun on the stone is the whole thermal mechanism, so a pile in shade is shelter and nothing more. What that stored heat does to nearby plants depends entirely on what limits growth where it stands. Where the season is short and frost ends it, the warmth on the sunny side extends the season for tender plants. Where summer heat and water are the limit, the same stone raises soil and air temperature, drives more moisture out of the soil and leaves, and stops the ground cooling at night, which stresses the plants closest to it. One mechanism, two opposite outcomes, and the site decides which.

It has to stay a discrete stacked object. Spread flat as a stone mulch it fills with windblown soil and weed seed, slides on a slope and becomes hard to rake clean, and sheet fabric laid underneath only restricts water and air and sends more of the rain running off. Upkeep is otherwise close to none, since unmortared stone has nothing that wears out. The one recurring task is keeping soil and leaf litter from closing the gaps, done sparingly, because animals take several months at least to find a new pile and disturbing it starts that wait again. Site it near standing water so what shelters there can drink, away from doors, foundations and where children play, away from stored feed it would otherwise hide rodents in, and apart from anything else kept for rodent control, which hunts the same prey and the same lizards.