Outdoor gardening in containers has lots of advantages, but it also has some challenges. One such challenge is watering correctly—essential for ensuring the health of your plants. While most people worry about under-watering plants, the reality is that it's just as easy to over-water and drown your plants with too much water.
Here are 10 tips for correct watering when gardening in pots and other containers.
Know Your Plants
Whether they are growing in the ground or in containers, the soil requirements for the vast majority of garden plants will stipulate "moist but well-drained soil." This means soil that isn't constantly soggy and saturated, just damp. Fortunately, with modern potting mixes that are designed for good drainage, this is easier than it used to be.
However, different plants have very different moisture needs. Some plants like to be dry, some like to be a bit dry between waterings, and then there are those prima donna plants that will swoon and drop all their buds and leaves if they get the least bit dry. However, as a rule of thumb, flowering annuals don't like to get too dry; succulents like to be a bit dry; and vegetables—particularly those that are juicy (tomatoes, cucumbers, melons)—like to be kept moist and need a huge amount of water. Some herbs (basil, rosemary, thyme, dill, oregano, cilantro) like to dry out a little between watering and the flavor will be stronger if they do. Other herbs such as parsley, sage, and chives like more moisture. One way to keep track of your plants' watering preferences is to keep the plant tag nearby, either under the pot or embedded in the soil.
Choose the Right Soil
Potting soil manufacturers offer several different soil "formulas" to simplify things for you. Products labeled as "general potting soil" are generally intended to provide that "moist but well-drained" quality that most plants prefer. Potting soils labeled "cactus and succulents" will be sandier soil that drains faster, providing an ideal environment for those plants that thrive on dry conditions. Other potting soil mixtures may announce themselves as designed for vegetables—these 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. They are said to "prevent against overwatering and under-watering," but in reality, they are best suited for plants that like quite a bit of water, such as vegetables and annual flowers.
Be aware that some potting soils will have control-release fertilizers added to them. There is nothing wrong with this, provided the nutrients are appropriate to the plants you want to grow. But if you use this kind of potting soil, you won't need to add water-soluble plant food when watering your plants.
Filling your pots with ordinary garden "dirt" is not very successful for a number of reasons—at least in part because garden soil does not have the moisture-retaining materials and good drainage found in commercial potting soils. While it is possible to successfully garden in containers filled with soil shoveled into them from the garden, it usually requires heavy amendment with materials such as peat moss, compost, vermiculite, or perlite.
Use the Right Containers
Many garden containers "breathe" and can allow the soil to dry out rather quickly. Clay terra cotta pots and coir moss hanging baskets are notorious for drying out quickly. Metal pots that capture the sun's heat can also quickly bake the potting soil dry. That's not to say you shouldn't use containers made from these materials, but it does mean you will need to watch them closely and water them more often than you would plastic or glazed ceramic planting containers.
Where practical, use the largest containers you can, holding the largest volume of potting soil. Large containers with lots of potting soil will hold more moisture and provide roots with ample space to soak up water. The smaller the pot, the more diligent you will need to be regarding checking the soil for moisture levels.
Check Moisture Levels
Before watering plants, check to see if your plant really needs it. The top of the soil can look and feel dry, even though it is quite moist just below the soil line. An easy test: Stick your finger into the soil all the way to the second knuckle. If it feels dry at your fingertip, your plants need water. Moisture levels can change quickly on a hot summer day, so a pot that feels quite moist in the morning may be dry by mid-afternoon.
The most important thing when watering plants is to give them a good, long drink—optimally until water runs out the holes in the bottom of your container. Depending on the size of your pot, many of the plant's roots will be down towards the bottom, and drenching the pot ensures that water will get all the way to the bottom roots. This practice also encourages roots to grow down toward the bottom of the pot, which is better for the plants. Frequent shallow waterings encourage plant roots to stay near the top of the pot, where they are more susceptible to heat and drought.
Water in the Morning
According to Horticulture Magazine, plants' roots are more receptive to watering in the morning and the evening and less so in the midday sun. Of the two options, the morning is the better time, since evening watering can allow water to sit on the leaves overnight, which can encourage fungal diseases such as powdery mildew. That said, if you get home from work and your plants are dry give them a good long drink, no matter what time it is.
Water the Soil, Not the Leaves
Some plants—especially those with hairy leaves—are susceptible to sunburn if you get water on their leaves in the sun. Water droplets can act like mini-magnifying glasses and burn your plant. Even if your plant's leaves are smooth, it is still a good idea to water the soil and not the leaves, if possible. Wet leaves can lead to an increased chance of fungus, mildew and other diseases.
Rather than overhead sprinkling, take the sprinkler head off the watering can or hose and apply water around the base of the plant.
Don't Rely on Rain
Even if you think that a rain shower has watered your plants, check the soil anyway. Sometimes a plant's foliage and flowers can act as an umbrella and actually keep water from getting to your soil, shedding the moisture right out of the container. And a short rainfall, even if it seems heavy, may not be nearly enough to fully saturate the potting soil, top to bottom.
Don't Let the Soil Dry Out Completely
Most potting mixes become tough and stop absorbing water efficiently if you let them completely dry out. Your potting mix can also pull away from the sides of your containers when it gets too dry, so while you may think you are giving your plant a good drink of water, the water may be just flowing down the sides of the pot and out the bottom, leaving your plant gasping for a drink.
If the soil does dry out, you have a couple of options. If your pot is relatively small, you can take the whole thing and submerge it in a larger container of water, taking it out when it has stopped bubbling. For a large pot or one that is difficult to move, poke holes in the soil with a pencil or skewer, and then give it a good drink, making sure the water is penetrating the soil and not just flowing down the sides.
Don't Assume Once Is Enough
Depending on where you live, the size of your pots, and the kind of soil you use, don't be surprised if you end up having to water your container gardens more than once a day. Heat, wind, and dry air can quickly parch your plants. Terra cotta pots, hanging baskets made from coir, and metal pots all can dry out ridiculously fast on a hot, windy summer day. Over the season, you will probably get to know which containers need to be checked more than once a day, but when they are first planted, it's a good idea to check your containers in the morning and again in the afternoon. A small pot may even require three waterings during brutally hot and dry weather.
Watering and Fertilizing Containers. University of Maryland Extension