The very best mulch is homemade compost.

I would research the walnut shell mulch. I don't know about the shells, but walnut trees are are known to inhibit the growth of other plants.

Here's an excerpt...

"Understanding allelopathy
Allelopathy is a complex phenomenon which Pliny the Elder, a Roman natural science author, first wrote about in 77 A.D. In his writings he noted the toxic effects of black walnut on neighboring plants in the landscape.

Allelopathy involves a plant's secretion of biochemical materials into the environment to inhibit germination or growth of surrounding vegetation. Allelopathy enhances tree survival and reproduction. Some plants that produce allelochemicals can be used in production as cover crops to control weeds. Researchers are presently attempting to breed crops and landscape plants that are allelopathic to weeds.

Allelochemicals are metabolic by-products of certain plants that, when introduced into the environment, cause growth inhibition by affecting physiological processes such as respiration, cell division, and water and nutrient uptake. Symptoms of "allelopathic effects" include leaf wilting and yellowing, or death of part or all of a plant.

Black walnut and allelopathy
Though grown primarily for its wood and nuts, black walnuts are often found growing on landscape sites where they serve primarily as shade trees. When certain other landscape plants are planted near or under this shade tree they tend to yellow, wilt, and die. This decline occurs because the walnut tree produces a non-toxic, colorless, chemical called hydrojuglone. Hydrojuglone is found in leaves, stems, fruit hulls, inner bark and roots. When exposed to air or soil compounds, hydrojuglone is oxidized into the allelochemical juglone, which is highly toxic.

Several related trees such as English walnut, hickories and pecan also produce juglone, but in smaller amounts compared to black walnut. Juglone is one of many plant-produced chemicals that can harm other plants in a process known as allelopathy. (Additional common landscape trees with allelopathic properties: sugar maple, tree-of-heaven, hackberries, southern waxmyrtle, American sycamore, cottonwood, black cherry, red oak, black locust, sassafrass, and American elm.) "

Here is a link that might be useful: the whole article

If you're want to feed your plants with the mulch then compost is the best. But, if you're just trying to keep the moisture in /or make it look nice then I like shredded cedar bark from any big box store.

We have used walnut shells for a play area and now have pecan shells for garden mulch. They hurt to walk on. : ) They are pretty and last a long time. They come with bits of nuts in them so that breaks down and you might get fungi or bugs or similar in them for a month or two. They are very cheap if you live near a processing plant.

Try woodchips or bark chips. Woodchips last a long time. Mulching controls weeds/plants where you don't want them to grow, slows down water loss from the soil, and enriches the soil as it slowly decomposes over a period of time.

I mulched with straw. Right now there are lots of pumpkin patches trying to get rid of their straw bales. You could get some for free if you ask nicely.

Aloha,

Nathan Lau
San Jose, CA
#include <std.disclaimer>

I have mulched with lawn clippings in my roses.

I've just bought Supersoil mulch in large bags because I always like the Supersoil products for cleanness and they don't have alot of fertilizers and additives.