Mulching your garden yields fertile and moist soil, and provides a good workout. Lifting and emptying bags of mulch and shoveling and spreading it can add up to a high-intensity or moderate-intensity workout.
The American Council on Exercise's calorie calculator identifies two levels of yard work: heavy and regular. Heavy yard work is more intense and burns more calories. While mulching, if you're breathing hard and need to pause every few words while talking to catch your breath, you're doing heavy yard work, according to the Centers for Disease Control and Prevention.
For example, if you weigh 175 pounds and do heavy mulching for an hour, you'll burn about 437 calories. If you do regular mulching for an hour, you'll burn around 318 calories.
You burn more calories the more you weigh. A 200-pound person would burn 499 calories doing heavy mulching and 363 calories doing less intense mulching, while a 150-pound person would burn 374 and 272 calories, respectively.
The figures given are only estimates and will naturally vary slightly for you according to your actual weight, health and fitness levels, muscle mass, intensity of your mulching activity and other factors.