Unless you receive sufficient rainfall, watering your outdoor planters is an important part of caring for your plants. Plants placed in containers such as planters have no way to receive water other that what is provided for them. Water requirements may vary depending on soil type, plant needs and planter type and size. A clay window box filled with impatiens will need watering more frequently and in larger quantities than a wooden whiskey barrel filled with petunias.
Check your soil daily with a moisture meter, which uses a probe to test the moisture content of soil. You can use the meter to determine if the soil is too dry for certain plants and water accordingly.
Fill a watering can full of cool water. Never use warm water. It will stunt the growth of roots. Use a large multi-gallon planter for watering your outdoor plants. A multi-gallon planter will help you to water several smaller planters or one larger container without having to refill your watering can often.
Add a few drops of a liquid, balanced (10-10-10) fertilizer to the water. Use the amount recommended by the fertilizer packaging. Liquid fertilizer concentrations vary among brands.
Tip the watering can on its side and allow water to run from the spout into the planter. Continue to fill the planter with water until a puddle forms on the surface of the soil and fills the container up to the brim. Hold the watering can upright and allow the water in the planter to drain completely away. Repeat this process a second time to ensure that the soil becomes saturated with water.
Observe the plant to ensure that the excess water drains completely from the soil. Many plants dislike being left in standing water. If the container continues to hold water, check the drainage hole to ensure that it has not become blocked.