You should use mulch because it protects garden soil in a few key ways. This extra layer between the ground and the elements protects against erosion, helps your soil maintain moisture, regulates changes in temperature, adds nutrients as it decomposes, and keeps weeds at bay.

The most important thing to put under a rock garden is the appropriate soil. The specific soil needed will depend on the type of plants you’re growing as well as the environment you live in. You can also put non-woven or woven fabric under the rocks to add to the garden's structural stability.

It depends. Smaller, younger trees take longer to mature, but they're more resilient, their roots don't get as damaged during transplanting, and they recover faster. Plus, they're much more affordable to plant, and it's possible to DIY the planting. 

On the other hand, mature trees produce fruit faster and provide more shade and privacy immediately. Ideally, a small or medium tree is the better choice if you can wait a few years for it to mature. But if you need immediate maturity, a larger tree is the only option.

Many people have used old tires in gardens as raised beds, planters, tree rings, and DIY decor. However, these uses are not recommended due to the toxicity of many of the chemicals used to manufacture tires. These chemicals can leach into the soil and water as the tire breaks down, creating environmental pollution and health hazards for gardeners and their families.