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Too much, not enough, too often or too seldom—these are concerns I hear from growers all the time regarding watering practices. The question most often heard, however, is simply, “How much?” A clearer understanding of the issues involved in watering your plants may help answer this for you.
In regards to how much is too much—when your soil has good drainage, your only real concerns are wasting water and draining off essential nutrients; otherwise, it’s not really an issue.
The optimal amount of water to apply is the amount you need to adequately saturate the soil to the depth of your plant’s root system. (If the plant is still in the early root development phase, the soil should be watered to a point a bit deeper than the roots currently exist.)
When the amount of water applied is insufficient or the watering time too short, the water does not extend all the way to the depth of the roots—this problem is common with soil that drains or perks slowly, and often results in a plant with shallow roots and poor yield.
Soil type has a huge impact on the amount of water you’ll need to provide your garden. No matter what the soil type you’re using, the entire root system should be wet when watering is completed. This process may take some time after the watering cycle has completed, as soil moisture osmosis can be slow and gradual.
In heavy soils like clay, a fully wet root ball can take several watering cycles to achieve because the water tends to run off if applied all at once or too quickly. This means that the optimal amount of water your plants require is also tied to the rate at which it is applied. When you apply water by hand, it tends to be applied quickly and conditions of unequal soil moisture are more likely to occur.
However, evenly moist soil provides your best chance for healthy root development and the maximum yield from your crop. The easiest way to achieve this is to use slow-drip watering, a process in which runoff is avoided and run time can be ascertained through the use of a soil probe and moisture meter.
Although moisture meters are by no means 100% accurate, they do provide a digital comparison between moisture levels at various depths of the root ball. This can add a lot of insight compared to the simple use of an “educated finger” for checking soil wetness.
All this can sound like way too much work in order to figure out how long to water, but it doesn’t need to be. With containers, potting soil is your typical growing medium. It provides good drainage, so if you have applied the water slowly, then the point at which you can observe runoff from the bottom of the container will indicate that you’re done.
Then, to determine when to water again, know your plant’s preferences and check your soil moisture periodically. Also, take notes—you won’t have to keep repeating this process if you are using the same containers and medium again.
Raised beds might have heavy, slow-draining soils, which introduce a variable of percolation rates and requires that water be applied slowly in order to prevent runoff at the top. Bottom runoff is not visible in raised beds, so a soil probe to examine the deeper levels of soil moisture will be required—in-ground garden beds are need this even more.
Now, let’s address an even more important issue: how often is too often? Most houseplants need water about once a week, but what about other crops? Watering a plant every day is frequently done, yet rarely debated; and while there are plants that do well with daily watering, there are many that do not. That’s why I feel a better understanding of watering frequency is vital.
The ideal condition of soil moisture for most plants is when the lower section of the root ball does not ever dry out, but stays moist while the upper section remains well-oxygenated and only slightly moist. (That being said, drought-tolerant species and low-water users are not tolerant of continually wet soil—their root systems need to have more oxygen present). Of course, when water is applied from the top—as is normal—then for a time the top section will naturally be the wettest. This is one reason why watering too frequently can cause trouble.
Factors such as evaporation, transpiration and soil drainage are all keys to figuring out a good watering schedule that will achieve the ideal soil moisture condition. Evaporation is a function related to the soil and is based primarily on ambient air temperature, wind and surface area.
When growing indoors, the air temperature is impacted to a great degree by the type of light source employed and the number of hours that the lamps are run. If you’re growing in a room without air conditioning, then evaporation will increase as the summer brings warmer temperatures.
Movement of water from the roots to the stems and leaves before exiting the pores or stoma is called transpiration. High air temperatures, dry air, wind and the type of plant involved all affect the rate of transpiration. When soil moisture is depleted and the roots can no longer pull up enough water, the rate will slow and the plant will start to stress. Leaves will flag or wilt and tissue death will eventually occur.
When trying to anticipate how much water may be needed for your plants, it should be apparent that mature plants with large roots will take up and give off much more water than small ones. Vegetative plants also use more water, in order to support production. In other words, you’ll need to increase watering frequency as the plant matures—especially when growing in a container.
Different soils also drain or percolate at different rates. Many potting soil products use coconut fiber to help retain water for a longer period of time, whereas other products add sand to help increase drainage and allow oxygen to return more quickly. Succulent plants do well with faster-draining soil, while bog plants like horsetail and canna lilies can have their roots wet all the time. Knowing plant and soil characteristics and then keeping that balance right during the various phases of plant growth is what makes for success.
A good balance between the air and water in the root system is also extremely important. Respiration is the process within a plant where oxygen is taken into the roots from the soil and the plant converts the sugars it created during photosynthesis into energy.
Without adequate soil oxygen (which can be depleted by overwatering), the roots do not function properly and the plant shuts down. Often the symptoms you’ll observe in this situation are almost identical to those you’ll see when a plant is drying out—the leaves flag or wilt and the plant begins to die.
This is why plants will actually die much faster from overly frequent watering than from drought. A comparison might be made with humans—how long can we go without breathing air, compared to how long can we go without drinking water?
Then, as briefly mentioned above, there are those plants that are drought tolerant—these varieties have adapted to growing in conditions where water may be withheld for weeks or even months. Many of these plants take up as much water as they can when it’s available.
So, when given irrigation frequently, these plants often take up so much water that it damages them. There is a big difference between low-water plants and drought-tolerant species, however—low-water plants just don’t take up water quickly and so will be far less affected by overwatering. Still, respiration is important for all plants and adequate soil oxygen must be assured.
So, the age-old question “How often and how much should I water my plants?” does not have a simple one-size-fits-all answer. Depending on plant characteristics and soil, proper watering frequency can vary greatly.
The key to establishing an appropriate routine is knowing your plants and your soil. If you understand the interaction between your soil, your plants and the ambient temperature, you’ll have a head start on a good watering schedule.
The use of a quality moisture meter can also help in monitoring and maintaining the correct frequency of irrigation. (The amount of dissolved oxygen in the soil is also great to know, but meters that measure this are rather expensive.) Follow these steps and, within a short time, your plants will be healthy and vigorous.
Source: Maximum Yield 05/11/18. "Water Plants: How Much Water is Too Much?" https://www.maximumyield.com/how-much-water-is-too-much/2/992