This appeared in the National Garden Bureau newsletter; an excellent description of how to water!
Timing: One commonly known fact is that watering in the morning is best. Not that you shouldn’t water in the afternoon or evening but when possible, avoid watering in the heat of the day. Morning watering is best for two reasons: 1) less water evaporates as you water and 2) Overnight dampness on the leaves could cause diseases so it’s better to be safe than sorry.
Frequency: Watering more thoroughly is better than watering more frequently. A thorough drink for your plants will help them establish deeper, stronger roots.
Amount: This is where it gets tricky and no one can precisely make a blanket statement on how much water your plants need. It depends on your soil type, the air temperature, wind, type of plant, age of the plant, in-ground vs. in containers (and which type and size of container). An overall rule of thumb is to give your plants 1” of water per week. If you have sandy or silty soil, you will likely need to water more than 1” per week. If temperatures spike then you will also likely need more than 1” of water per week. For vegetables, here are a few more specific guidelines:
* All seeds need even moisture during germination.
* Beans need more watering when they’re flowering.
* Sweet corn needs water during silk, tassle and ear development
* Watermelon needs more water during fruit set and growth
* Tomatoes need consistent amounts of water to prevent blossom end rot