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One of the first things we learn about growing plants is the importance of watering to their health and ability to absorb nutrients. Improper watering is one of our first suspects when a plant is ailing.
But deciding how much and how often to water a plant can be tricky. Various terms are used to describe watering requirements, and climate and other factors can complicate the decision even more.
So how can you decide how often to water your plants? Read on to find out.
No Blanket Amount
It is important to understand that there is no blanket or generalized watering amount or frequency that can apply to all plants or even to broad categories like all indoor plants, all plants in a given region or hardiness zone, and so on.
Different plants have different watering needs, regardless of our schedules. Of course, you can choose which plants to grow – or to grow near each other, also known as companion planting – based on similar watering needs. In fact, in most cases, that is a good idea!
(By the way, if you're interested in getting a FREE printable garden planner and learning more about companion planting, enter your email address in the form below. One of my follow-up emails will discuss how to decide about companion plants for any plants or pairs not yet covered in our gardening facts articles: )
But let's say you grow lettuce, marshmallow, thyme, and aloe vera, and you decide to water all of these plants deeply once per week because that is what works best for your schedule. As you can see from clicking on the links for each of those plants, each one has vastly different watering needs.
So the thyme might be fine or might be slightly overwatered, depending on other conditions. But the aloe will definitely be overwatered, and the marshmallow will almost certainly be underwatered. This can mean poor plant health and eventual plant death for both species if it is not corrected.
Looking at lettuce's range of 1-2 inches per week, you might think it would be fine with this watering schedule. But since lettuce has shallow roots, it is better to water it in smaller amounts multiple times per week rather than doing 1 deep watering once per week. Otherwise, it could suffer poor health, poor quality, and eventual death as well.
So besides the simple recommended weekly amount, there are other factors to consider. We will discuss these more later in the article, but let's first review the common types of watering instructions.
Types of Watering Instructions
As you read watering instructions, whether on our site, on your seed packet, or elsewhere, you may come across several different terms and ways of communicating how much water a plant needs.
You may see something like this statement found in the Growing Tomatoes: Quick Gardening Facts article: "Tomatoes need 1-2 inches of water per week." In this example of measuring water needs in inches per week, 1 inch per week means that the water should cover the entire area of the soil where the plant roots are located to a depth of 1 inch.
For outdoor plants, an easy way to measure the rainwater your plants receive is with a rainwater gauge like this one. From there, you can calculate the difference between how much water your plant needs and how much it has already received via rain. So if your plant needs about 1 inch per week and it has received 0.5 inch of rain that week, you would need to water that plant another 0.5 inch yourself, whether that is through drip irrigation, hand-watering with a watering can, or some other means.
NOTE: Remember to water the entire area where the plant's roots are, not just the base of the plant. This is especially important with plants whose roots grow widely such as cauliflower, aloe, and mints.
The inches-per-week method makes it easy to quantify how much to water your plants when weather conditions and other factors are fairly stable and consistent.
Many plants whose water requirements are expressed this way use a range, as in the tomato example above. Deciding how much to water within that range requires considering other factors and conditions, which we will review in the next section.
Another way of expressing watering needs is something like this example from the Growing Oregano article: "When the top half-inch of the soil has dried out, water the oregano deeply." Deciding how much to water by the moisture level of the top part of the soil is an excellent route because it has already taken many of the climate factors discussed below into account.
Moisture level can be measured by simply sticking a finger or two into the soil to determine whether or not it has dried out. Whether the top inch, half-inch, 2 inches, or other amount needs to be measured varies by plant and should be indicated in the instructions.
Of course, the direction to "water deeply" can seem vague. This phrase is used both in moisture-level instructions, as in the oregano article above, and others that have it as part a frequency instruction, as in this example from the Growing Thyme article: "Thyme should be watered deeply once every 1-2 weeks."
The idea behind watering deeply is that the water reaches all – or, at least, as many as possible – of the plant's roots. This is accomplished by watering slowly, letting the water sink in, and repeating this until the roots are saturated. Watering too quickly can cause the water to flood the plant base and upper roots and to run off to the sides rather than actually being absorbed into the soil. This is even more true when the soil starts off dry.
For shallow-rooted plants like spinach and lettuce, watering deeply might only include total amounts of 2-4 inches. For deeper-rooted plants, it could be 10 inches, 12 inches, or more.
If you are unsure of your plant's root depth or if it has a very deep taproot that is unlikely to ever be fully saturated, water to a depth of at least 8 inches.
If your mature plants are part of a container garden, you can tell that your plant is watered deeply when the water runs out of the drainage holes in the bottom of the planter. This is a perk of container gardening! Be mindful, though, that container-grown plants are likely to dry out more quickly than in-ground ones.
(By the way, learn how to choose the right planter for your container garden here!)
Which terminology is used often depends on available data, experience, and writer/instructor preference. For some plants, information available for multiple methods. For example, our Growing Sunflowers article gives amounts in both inches per week and moisture level.
Incidentally sunflowers are excellent examples of plants with very long taproots that are best watered to a depth of at least 8 inches. A taproot is a long, skinny root that usually goes straight down, so typically watering is only needed around the base of the plant, unlike plants whose roots spread more widely. An exception would be container-grown sunflowers since the container limits the taproot's length and encourages it to wind around once it runs out of vertical space.
Factors to Consider
Some factors to consider are climate, humidity, weather, and root depth. Of course, this is not an exhaustive list but a good starting point.
Climate and region are important because some climates, such as desert regions and high elevations, tend toward dryer soil. This means plants will likely need to be watered on the higher end of inches-per-week and frequency ranges.
On the other hand, places in lower heat zones such as the Pacific Northwest coast and the Upper Peninsula of Michigan need to factor in milder/lower summer temperatures than most of the rest of the US. These lower temperatures cause water to evaporate more slowly. Learn more about heat zones here.
Of course, in the case of the PNW coast in particular, the 3+-month-long drought we experience in mid to late summer every year also means that we cannot rely on natural rainfall, unlike with spring, late autumn, and winter crops.
Other climate and regional factors may exist in your area. Try to think of what makes your area's climate different from others in your country, continent, or other regional division and how that might make the air and soil moisture levels differ from the general watering recommendations found here, on your seed packet, or elsewhere.
Humidity is another factor. Plants in higher humidity do not need as much water since they absorb some moisture from the air. Watering the soil is still important, but water requirements are likely to be on the lower end of a range.
Of course, humidity is included in climate and regional variations in some ways, as summers are very different in Florida than they are in Nevada, for example. But humidity can also vary within the same region from season to season, month to month, week to week, and even day to day. Conditions such as droughts and unusually frequent storms can make the humidity level much different than normal. So be sure to be mindful of both climate-scale and day-to-day humidity.
Indoor humidity affects indoor plants similarly to how outdoor humidity affects outdoor plants. Be sure to take humidity into account if you garden indoors as well.
Along a similar line of reasoning, weather can impact how much and how often you water your outdoor plants. Natural rainfall can mean less supplemental watering for you, but extremes like flooding and intense storms can be harmful to your plants, especially those that need dry and non-waterlogged soil. Obviously lack of sun can cause other issues with your plants, especially those that require full sun, but a string of cloudy days without rain could still mean you don't need to water as often since the sun is not there to speed up evaporation.
This includes temperature as well. In hot temperatures, plants need to be watered more often than in cool temperatures.
The final factor for our discussion is root depth. As mentioned in the lettuce example above (click to read it again), in most cases, plants with shallow roots benefit from shallower, more frequent watering sessions. Plants with deep roots need to be watered more deeply.
And there you have it. Happy growing and watering!