University of Illinois Extension
Watering - Bed Preparation and General Maintenance - Stepping Stones to Perennial Garden Design - University of Illinois Extension

Watering

Remember, an ideal garden soil is 25 percent moisture. This is supplied by rainfall and in dry times, by you! It is very difficult to tell someone how to water their garden. Every location is different due to soil type, environment and topography. The standard answer is, your gardens should receive one inch of water per week during the growing season. If this isn't supplied by rainfall, you will need to water the garden.

Always check the soil first before assuming a garden needs water. A wilting plant doesn't always indicate a need for water. Use a trowel and check down about 4-6 inches, where the roots are. If you just brush and feel the upper inch, you are not getting an accurate assessment. Water deeply. Set out a rain gauge or an empty tuna fish can marked at 1-inch. Set your sprinkler and let it run until one inch of water accumulates. Research has shown that deep, infrequent watering is better for plants than light, frequent watering. One inch of water will percolate into the upper 6 to 8 inches of an average soil.