Hello Valerie,

That is an excellent question!  The answer (as with so many things in life) is it depends.

Some rock functions very well as mulch, improving water infiltration and retention, and preventing weed growth.  To get all the benefits of mulch, rocks need to be small--1/2" across or so.  Larger stones can be decorative but don't provide the advantages of mulch.

All rocks, regardless of size, will transfer heat to the soil below, which can be either a benefit or a potential problem, depending on the types of trees or shrubs you are growing.

A disadvantage of rock under trees and shrubs is that it can be a hassle to keep looking clean, since debris from the plants will collect and cover them.

Wood chip mulch is excellent at preventing water evaporation from the soil and suppressing weeds.  It can cause problems for water infiltration if it gets dry.  Bark mulch, particularly "gorilla hair" products, are famous for causing water to run off rather than soak in, so these mulches are best used with drip irrigation emitters installed below the mulch.  Wood, bark, or "green waste"--chopped landscape debris, essentially--all help insulate the soil and keep it cool, which again can be helpful or harmful, depending on the plants you're growing.  They have the advantage of being able to naturally accommodate debris and leaves, or at least don't need meticulous cleaning like a stone mulch would.

In general, wood chip mulch (not bark) is probably ideal in most situations for trees and shrubs.  For fire safety, do not use it within 5 feet of your home or other structures.

More information about various mulch options can be found here: https://extension.colostate.edu/docs/pubs/garden/07214.pdf

I hope this is helpful!

Cordially,

John

Thank you John for all this great information! I will pass it on..  Valerie 
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