Cocoa Mulch contains a lethal ingredient called "Theobromine".

It is lethal to dogs and cats. It smells like chocolate and it really attracts dogs. They will ingest this stuff and die.

Just a word of caution — check what you are using in your gardens and be aware of what your gardeners are using in your gardens.

Theobromine is the ingredient that is used to make all chocolate — especially dark or baker's chocolate — which is toxic to dogs.

Cocoa bean shells contain potentially toxic quantities of theobromine, a xanthine compound similar in effects to caffeine and theophylline. A dog that ingested a lethal quantity of garden mulch made from cacao bean shells developed severe convulsions and died 17 hours later. Analysis of the stomach contents and the ingested cacao bean shells revealed the presence of lethal amounts of theobromine.

This warning about the potential danger to pets posed by cocoa mulch began appearing in May 2003. Unlike the majority of scary alerts spread through the Internet, there is a good deal of truth to this one.

Cocoa mulch contains ingredients that could pose a health risk to dogs (and other pets that might be tempted to ingest it):

Found in most home garden centers, cocoa mulch is known for its fine texture and the sweet smell the fresh mulch gives off.

But getting past the scent, cocoa mulch can be dangerous if a dog starts eating it. It contains two key ingredients found in chocolate: theobromine and caffeine. Similar to eating chocolate, a dog that eats just a few ounces of cocoa mulch could start having stomach problems and it could get worse if it eats more.
As time goes on they might act restless, excited, it can produce tremors and seriously seizures.

Puppies are very curious animals. So they're going to be attracted to various things around the yard and the effect of eating cocoa mulch seems to be more severe in the small breeds.

Rather than gamble their dogs won't be attracted or harmed by the mulch, responsible pet owners will probably prefer to choose another form of soil enhancement for their gardens, such as cedar-based products.

The danger of canine theobromine poisoning does not begin and end with cocoa mulch, however: chocolate in any form poses substantial risks to some pets. This most beloved of foodstuffs contains theobromine and small amounts of caffeine, both of which can sicken and even kill cats and dogs.

The American Society for the Prevention of Cruelty to Animals (ASPCA) confirms the potential effects of theobromine and caffeine on dogs:

Cocoa beans contain the stimulants caffeine and theobromine. Dogs are highly sensitive to these chemicals, called methylxanthines. In dogs, low doses of methylxanthine can cause mild gastrointestinal upset (vomiting, diarrhea, and/or abdominal pain); higher doses can cause rapid heart rate, muscle tremors, seizures, and death.
Eaten by a 50-pound dog, about 2 ounces of cocoa bean mulch may cause gastrointestinal upset; about 4.5 ounces, increased heart rate; about 5.3 ounces, seizures; and over 9 ounces, death. (In contrast, a 50-pound dog can eat up to about 7.5 ounces of milk chocolate without gastrointestinal upset and up to about a pound of milk chocolate without increased heart rate.)

Chocolate's toxicity to animals is directly related to three factors: the type of chocolate, the size of the animal, and the amount of chocolate ingested. Unsweetened baking chocolate presents the greatest danger to pets because it contains the highest amount of theobromine, approximately 390-450 mg. per ounce. White chocolate contains the least. As a general rule of thumb, one ounce of milk chocolate per pound of body weight can be lethal for dogs and cats. (Milk chocolate contains approximately 44-66 mg of theobromine per ounce).

Theobromine affects the heart, central nervous system, and kidneys, causing nausea and vomiting, restlessness, diarrhea, muscle tremors, and increased urination. Cardiac arrhythmia and seizures are symptoms of more advanced poisoning. Other than induced vomiting, vets have no treatment or antidote for theobromine poisoning. Death can occur in 12 to 24 hours.