Nothing frustrates a new gardener more than the watering issue, largely due to the fact that the signs for over watering are very similar to those for under watering.
€ Yellowing and/or wilted leaves
€ Plants not thriving i.e. no new growth
€ Brown needles in evergreens

The best way to know if your plant needs water is to feel the soil around it, even dig down and turn a bit of the soil. Some plants do require more water than others especially when first planted (i.e. hydrangeas vs. ornamental grass). But if the ground around the plant is wet it doesn't need more water. It is also better to water deeply less often than watering lightly every day. This encourages the root system to grow deep instead of staying at the surface.

The roots are the most important to the plant (and often most forgotten) because they are its primary source of water and food and are also important for the uptake of oxygen. The roots of the plant take up water but they also need air to breathe. Over-watering, in simple terms, drowns your plant. Soil that is constantly wet won't have enough air pockets and the roots can't breathe.

Some key things that will help you be successful: Quality plants, good soil and mulch.

Quality plants are important, if the plants are growing in a pot of poor soil and not fertilized regularly at knowledgeable nurseries, chances are they will have weaker roots and therefore more likely to cause you more maintenance in your garden. You really do get what you pay for in plants.

It is also important to have good soil. Planting in soil with poor drainage like clay or too much drainage like sandy soil can make meeting the needs of your plants even more challenging. If you think your soil is poor in quality then adding compost to the soil is the best way to resolve it. Adding €more€ soil is not the answer. Compost will breakdown and improve the existing soil which will also provide nutrients for the plants.

Adding a good layer of mulch will also protect the plants roots especially in hot weather. The soil under mulch will not dry out as quick.

If you feel you have been over watering a plant, try holding back on the water, keep checking surrounding soil. Water sparingly as needed and when you see signs of new growth add a water soluble fertilizer to give the plant some nutrients.