Here are ten tips and best practices for watering outdoor container gardens.
Know Your Plants
Whether plants are growing in the ground or in containers, soil requirements for the vast majority of garden plants stipulate "moist but well-drained soil." This means soil that isn't constantly soggy or saturated, able to drain well so that soil is slightly moist. Fortunately, with modern potting mixes that are designed for good drainage, achieving this is easier than it used to be.
We all know that different types of plants can have different requirements for soil moisture. Some plants like to be dry, some like to be a bit dry between waterings, and then there are those that will swoon and drop all their buds and leaves when they are the least bit dry.
As a rule of thumb, most flowering annuals don't like soil conditions to become too dry; succulents prefer the soil to be a bit dry; and vegetables—particularly those that are juicy (tomatoes, cucumbers, melons)—like soil to be kept consistently moist.
Some herbs (basil, rosemary, thyme, dill, oregano, cilantro) do best when soil dries out between waterings, it enhances their flavor. Other herbs such as parsley, sage, and chives like more moisture.
One method to keep track of a plant's moisture requirements is to save and keep the plant tags nearby, either in a plastic bag, folder or binder, under the container itself, or embedded in the soil next to the plant.
Choose the Right Soil
Potting soil manufacturers offer several different soil formulas to simplify things for you. Products labeled as "general potting soil" are intended to provide that "moist but well-drained" quality that most plants prefer. Potting soils labeled for "cactus and succulents" contain sand that drains faster, providing an ideal environment for those plants that thrive in dry conditions. Other potting soil mixtures might be advertised as being suitable for growing vegetables—these mixes are formulated to absorb and hold water somewhat better than standard potting soils.
Soils labeled "moisture control" typically have a higher percentage of peat moss, coir, and other wetting agents. Reportedly they "prevent overwatering and under-watering," but in reality, they are best suited for plants that require moist soil, such as vegetables and annual flowers.
Be aware that some potting soils have time-released fertilizers added to them. There is nothing wrong with this, provided the nutrients are appropriate for the plants you want to grow. If you use pre-fertilized potting soil, you don't have to fertilize your container with water-soluble plant food.
Filling containers with soil from your garden beds or purchasing commercially-sold products labeled as "garden soil" is not recommended for containers for a number of reasons—garden soil does not provide adequate drainage or aeration, it's too heavy and dense, it compacts easily, and soil taken directly from garden beds can contain contaminants such soil-borne diseases, insect pests, and weed seeds.
Use the Right Kind of Containers
Many garden containers are porous, which can cause the soil to dry out rather quickly.1 Terra cotta containers and coir hanging baskets are notorious for drying out quickly. Metal containers can also dramatically increase soil temperatures in containers, quickly drying soil and baking your plants. You can use containers made from these materials, but you must monitor them closely and water them more often than you would plastic or glazed ceramic containers.
Where practical, purchase the largest containers you can that are appropriate for the area where you will place them. The general rule of thumb is that outdoor containers should be no smaller than ten inches in diameter; any smaller and they dry out too quickly and the containers can quickly become root bound.
Larger containers hold more soil and moisture to provide roots with enough space to grow and absorb water and nutrients. The smaller the pot, the more diligent you need to be monitoring soil moisture levels.
Check Moisture Levels
Before watering a container, be sure that the plants need water. Over-watering is just as harmful as under-watering. The soil at surface of the container might look and feel dry to the touch, but the soil might be moist just an inch or two below the surface.
To test container moisture, try this: stick your finger into the soil as far as it goes or at least to your second knuckle. If the soil feels dry at your fingertips, the plants need water. Moisture levels can change quickly on a hot summer day, so a container that feels quite moist in the morning might be dry by mid-afternoon.
The most important thing to remember when watering containers is to water deeply—this means that you should see water running out of the drainage holes at the bottom of the container. If you don't see watering flowing from the bottom of the container, you have not watered enough..1
Healthy root systems quickly grow and branch out through the soil towards the bottom of the container. Drenching the container ensures that water reaches the entire root system. Watering deeply encourages plants to develop strong root systems, and that provides better nutrition for the plants.
Frequent shallow waterings encourages roots to remain near the soil surface where they are more susceptible to heat and drought and with less ability to absorb nutrients that are available deeper in the container.
Water in the Morning
According to Horticulture Magazine, plants are more receptive to watering in the morning and less so in the midday sun. Morning is ideal because it provides sufficient moisture to the plants for the entire day, there is less evaporation caused by wind and heat, and it allows wet foliage to dry out before nightfall.
Watering in the evening is not recommended because the foliage does not have enough time to dry before the sun goes down. Wet foliage can invite fungal diseases such as powdery mildew.
However, not everyone has the opportunity to water containers in the morning given time pressures of getting children to school or heading off to jobs. If you can't water containers in the morning, water them when they're dry no matter what time of day it is.
Water the Soil, Not the Foliage
Plants absorb water through their root systems, not through their leaves, stems, or flowers. Thus, to properly water your containers, apply water to the soil where it will reach and be absorbed by the plant roots. Wetting foliage can lead to an increased chance of fungal and other diseases and the water is wasted anyway.
Another reason to keep foliage dry when you water is that some plants—especially those with hairy leaves—can be susceptible to sunburn in the hot sun. Water droplets attach themselves to tiny hairs, and when water collects on them, it's possible that the water droplets act like mini-magnifying glasses. Sunburn won't occur on smooth-surfaced leaves.
Don't Rely on Rain
Even if you think a rain shower has watered your containers, don't be so certain because it's usually not true. Plant foliage can act like an umbrella and actually prevent water from reaching the soil. With containers filled with mature plants, soil might not even be visible so it's impossible for rain to penetrate the thick growth. Rainfall amounts, even those from a heavy storm, might not be nearly enough to fully saturate container soil from top to bottom. Take matters into your own hands and monitor container moisture yourself, even after a heavy rainfall.
Don't Let the Soil Dry Out Completely
Most potting mixes become tough, hard, and stop absorbing water efficiently if you let them completely dry out. Dried out potting mix can also pull away from the sides of containers. So while you might think you are applying enough water, the water might be flowing down the sides of the container and out the bottom, leaving your plants gasping for water.
If the container soil dries out, here are two methods to re-hydrate them:
If the container is relatively small, fully submerge it inside a larger container or sink filled with water. Remove the small container when it has stopped bubbling.
If the container is large and is difficult to move or lift, poke a few holes into the soil with a pencil or skewer. Then apply a gentle, slow stream of water to the soil making sure the soil is absorbing the water. Repeat this process until the soil is softened and fully moist.
Don't Assume Once Is Enough
Depending on the climate, the size of your containers, and the kind of soil you use, don't be surprised if you have to water your containers more than once a day.
Heat, wind, and dry air can quickly parch your plants. Metal and terra cotta containers and hanging baskets made from coir can dry out ridiculously fast on a hot, windy summer day.
Over the growing season, you will know which of your containers and hanging baskets dry out the fastest. When you first plant your containers, monitor them for moisture in the morning and then again in the afternoon to see which containers might require more watering than others. You might find that watering once in the morning simply is not enough. A small container (ten inches or less in diameter) might require three daily waterings during brutally hot and dry weather.
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