Garden plants such as tomatoes, watermelons and some flowers like a lot of water and prefer not to dry out much in between waterings. These plants are the first to suffer if you must leave them, unwatered, as you go on a trip out of town, or if hot dry weather or poor soil conditions rob them of necessary moisture. You have many products/methods available for keeping garden plants moist. It's just a matter of choosing the best watering system and soil amendments (if necessary), and planning ahead.
Add moisture-retaining additives to the soil, such as peat moss, if necessary. Sandy soil is especially prone to poor moisture retention.
Give plants a good soaking in the morning, before temperatures have started to rise. This will help the soil stay moist longer--watering in the morning gives the plants time to absorb water before it evaporates. Watering in the late evening will do the same; however, too much moisture in the soil overnight can cause fungal diseases.
Add mulch around the plants. You can use wood chips, straw, sphagnum moss, plastic mulch (available at garden supply stores) and other materials. Mulch keeps plants warmer and slows water evaporation from the soil.
Use a soaker hose to water plants slowly. Unlike conventional garden hoses, soaker hoses contain small holes throughout their length. If you place these near your plants, water seeps into the soil at ground level, over time. These hoses can be especially helpful if you will be away from home for a day and your plants are very thirsty plants.