Well if you're using cocoa mulch, it could be lethal to your pets. The "cocoa" bit should be a dead giveaway with more people becoming aware that chocolate (milk and dark chocolate particularly) is poisonous to pets.
What we're dealing with here is Theobromine, a xanthine compound found in cocoa bean shells that has a similar effect to caffeine (in fact, it's a by-product of the body's metabolism of caffeine). It's one of the things that makes chocolate such a great comfort food - but unfortunately also the key reason chocolate (and cocoa mulch) is so dangerous for your pet if ingested.
To give you an idea of how it impacts an animal, theobromine is - according to Wikipedia - used in modern medicine as a vasodilator (a blood vessel widener), a diuretic (urination aid), and heart stimulant.
The problem for animals - particularly dogs, who are more likely to eat chocolate (or that yummy-smelling cocoa mulch all over your garden) than cats - is that they metabolize theobromine way more slowly than humans. In fact, an animal can succumb to theobromine poisoning from as little as 50 grams of chocolate for a smaller dog and 400 grams for an average-sized dog.
Wikipedia lists complications including "digestive issues, dehydration, excitability, and a slow heart rate. Later stages of theobromine poisoning include epileptic-like seizures and death. If caught early on, theobromine poisoning is treatable but ,while rare, fatalities can and do occur.
So do your research - natural doesn't always mean safe. It's true that most dogs won't go on a cocoa mulch binge, but do you really want to take that chance for the sake of a lovely garden?
Resources and references:
- http://www.snopes.com/critters/crusader/cocoamulch.asp - confirmation that cocoa mulch is, indeed, dangerous to pets. Includes some useful fact-checking research.
- http://www.growingwisdom.com/index.aspx?pid=7&sid=1 - some videos about safe organic composting and selecting safe mulch.
- http://www.petplace.com/dogs/cocoa-mulch-toxicity/page1.aspx - article on cocoa mulch toxicity with tips for treatment and prevention.
- http://www.diynetwork.com/diy/gr_general_info/article/0,2029,DIY_13847_4615714,00.html - DIY network article on pet friendly gardening and landscaping.