I was told tea tree mulch could poison my pet dog and cat if in contact with this mulch.
Is Tea Tree Mulch Poisonous to Dogs
I can't find anything in the literature that suggest tea tree (melaleuca) mulch is poisonous to dogs and cats. Tea tree oil in high concentration can be dangerous; this may be how the tree mulch became associated with toxicity. The only mulch that is consistently listed as toxic is cocoa-shell mulch. Of course, any kind of wood mulch that is ingested frequently by a dog can make them sick; if you have a dog that just won't leave the mulch alone, you might want to consider preventing them from reaching the mulched area, or using large river stone mulch (even small gravel can cause damage to a dog if it's swallowed.) I did see one dog-breeder forum that recommended using tea tree mulch. This article has some more general information on mulches that might be useful to you: https://www.gardeningknowhow.com/garden-how-to/info/whats-the-best-mulch-for-my-garden.htm