What is the difference between shredded mulch from free vs garden center sources?

Question

Many municipalities use brush collection materials to create mulch, which they supply for free to their taxpaying citizens.

On some gardening sites, users have chimed in that these free mulch sources are not safe (safe for your desired landscaping or garden results, not a human danger) for home lawn and garden use, since the wood that winds up there are often from diseased sources that get mixed in with the rest of the mulch.

Is this a legitimate risk? It seems very much like the kind of warning that might originate from a garden center store that sells much more expensive mulch.

If that is the danger of free brush-collection-sourced shredded mulch, where do commercial mulch sources get their materials, and what precautions are used to insure their comparative safety?

Some possibilities I thought of -

1. No difference at all. They get their source material from pruned trees, removed trees and stuff that falls into yards from trees. Maybe they even buy the brush collected by cities that don't shred and supply mulch.
2. They get all the limbs, branches and odd bits, either pre- or post-shredding from lumber mills, that trim from the main trunk of threes they are milling for lumber.
3. They grow the most pristine, perfect and healthy trees themselves, just to cut them down and shred them into "pure" mulch (/sarcasm)

If anyone actually knows, generally, how that works, I'd be interested. It's entirely possible my skepticism is influenced on my preference for "free."

EDIT - I made some assumptions in language and not everyone is probably familiar with "brush collection." Here is a municipality's site on what qualifies and how it is done -

Madison, Wisconsin yard waste brush collection info

Answer 1

Since your question deals with "shredded mulch" I'll limit my response to that.

The main differences, as far as I see things, are:

• Municipal mulch can be free - I like free.
• Bagged mulch from garden center has likely traveled a long distance and created a larger carbon footprint as a result of shipping.
• Bagged mulch from garden center is likely using poorer source materials than the municipal mulch - UNLESS it specifically identifies the type of base material. Often commercial operations use recycled wood which can contain treated lumber, which is bad.
• Shredded mulch at OUR municipal facility does not go through any intentional hot process. It does, however, spend a fair amount of time in piles and I have seen these piles steaming on cold days - so there is some hot process occurring.
• The bagged mulch from the garden center may contain dyes - black and red being the most common. Avoid these at all cost.

I'm out of time, but might add more later.

Answer 2

The answer rather depends on what material you mean by 'mulch' (other than the general description of any material spread over the surface of soil), and whether your municipal body uses a hot, aerobic or cold, anaerobic system to produce whatever it is.

It seems as if many municipalities in USA do produce leafmould, but I'm not sure what else they produce. In the UK, you can buy (it's not free here any more) composted material from a few local council authorities which can be used as a mulch or soil conditioning compost. The best are produced by using a closed, hot method, with batch mixing and testing over time to check for heavy metals, etc., to ensure its safety and quality, and they are produced to a British Standard. The standard is so high that many local authorities here don't supply to the public any more, they send their green waste to a waste processing plant which uses a proper hot system and get paid for the waste.

Using a hot method destroys weed seeds, roots and pathogens, so you should check with your own local authority precisely what they have for free, and how it's produced. If it's cold produced, it is likely to contain both pathogens and weed seeds.

I don't know what you would have over there as an equivalent of a British Standard number or quality mark, I'd have thought there'd be something. Buying similar materials at the garden centre is often more expensive than from a local authority, but here, anything described as (organic) 'mulch' is usually bark chips or cocoa shells, and composted materials are sold as soil conditioning compost or potting compost if its sterile enough, though there's no reason why soil conditioning compost can't be used as a mulch, it usually is to save digging in.