How often should I water? This is one of the most common questions new gardeners ask. It sounds like a good question, and on social media, lots of people will supply an answer.
They are all wrong.
Why We Water
Plants need to be able to get enough water through their roots to keep the top green parts growing properly. They are able to do this when they have a good root system and the soil around the roots has enough water in it.
When water is added to soil, it fills the small pore spaces between soil particles and stays there until it runs away, plant roots absorb it, or it evaporates. Over time the soil becomes too dry and roots can no longer find enough water. The garden should be watered before this happens, but how does the gardener know when this critical point is reached?
Rate of Evaporation
To understand watering, it is important to understand evaporation. The rate of evaporation is a term used to describe how quickly water evaporates. Many things affect the rate of evaporation.
- Temperature – evaporation is higher at higher temperatures.
- Type of soil – clay holds lots of water, sand holds very little, and peat moss is somewhere in between.
- Size of pot or container – large pots tend to be deeper, and deeper soil holds water better since evaporation takes place at the surface of the soil.
- Type of container – clay pots are porous and have higher rates of evaporation than plastic pots.
- Humidity – evaporation is slower in humid environments than dry environments.
- Mulch on the soil surface reduces evaporation significantly.
- Wind increases the evaporation rate.
Watering on a Schedule
Everyone wants to water on a regular schedule: once every three days, once a week and so on. This does not work. If you look at the above factors affecting the rate of evaporation, you quickly realize that they change all of the time. Summer is hotter than winter. Spring may be drier or more humid than fall, depending on where you live. Humidity can change daily as different weather systems move through.
Plants also affect the amount of water in soil. Dormant plants remove water more slowly than actively growing ones. Plants that are located close together remove water faster than if they were planted farther apart. A perennial in early spring has few leaves and the amount of water transpired through the leaves is small. By mid summer the same plant is covered with leaves and it is warmer; resulting in high transpiration rates.
I frequently see people on social media asking for a watering schedule, “I just bought a new rose, how often should I water it?” Other people can’t tell you how often to water because they have no information about the factors affecting your rate of evaporation or transpiration. Only you can figure out how often to water.
What Time of Day is Best?
What is best, morning, noon or night? I had a close look at this question in Best Time to Water – Morning, Noon or Evening?
Wilting Plants
Wilting plants may or may not indicate that it is time to water.
Leaves will wilt if the soil is too dry and roots can’t get enough water for the plant. In this case it is a good idea to water.
If you water too much, the roots die, and the plant can’t get any water, which results in wilting plants. The last thing this plant needs is water.
For more on this topic see Should You Water When Plant Leaves Wilt?
When Should You Water?
The simple and correct answer is, when the soil needs it. It is really that simple.
How do you determine if the soil needs water? Use the finger meter. Stick your finger into the soil a couple of inches, and if it feels dry, water. If it feels moist, don’t water.
This works for house plants, outdoor planters and the garden.
Let me give you a simple example. Newly planted trees should be kept moist all of the time for at least a year. I usually plant in fall, in clay soil which holds water quite well. I also mulch with about eight inches of wood chips. If I water well in late September, I don’t have to water again until next summer. Zone 5 cools down in September, reducing evaporation. The mulch reduces this even more.
How could anyone on the internet tell me when to water?
Water when soil needs it – not on a schedule.
Tips and Tricks for Watering Plants
Here are some more posts about watering plants.
Watering Houseplants – Top or Bottom? Which is Best?
Watering Plants in the Sun – Do Water Droplets Damage Leaves?
What is the Best Watering Schedule for Your Garden
Watering Plants Correctly – When and How to Water
Watering Orchids With Ice Cubes – Does It Harm orchids?
Thank you for your post. I really appreciated the part about watering when the plants are wilted. I too live in zone 5 and we recently went through a period of 90° temperatures with heat indexes at or above 100+ degrees. If I saw my squash plants, which are in raised beds, had wilted leaves between the hours of 1 and 3 pm, I would check the soil (there are many reasons I check the soil – even if I had just watered the day before), and if it felt dry I would water the ground again. I can’t tell you how many times my aunt who lives with me and others would tell me I shouldn’t be watering during the hottest part of the day. Doing so can burn the roots. At least that is what they all learned on YouTube. I figured common sense says your plant is stressed, the soil has dried out again, water the plant if the leaves are drooping. If I find the soil is still moist, then I know to leave them alone, and that the drooping is the plants way of conserving moisture and once 5 pm comes around the leaves will bounce back. The other thing I check when I see the leaves start to droop on the squash plants is the base of the stem. If a vine bore has made its way into the stem then that is the cause and not the heat or lack of water. 🙂. I would love to hear your thoughts on my reasonings of watering during the heat of the day.
Nothing really wrong with watering in the heat of the day if the soil is dry.
Great post as always! But I noticed you didn’t mention my favorite way to determine when to water plants in pots, and that is by the weight of the pot. Just by lifting the pot you should be able to estimate the amount of water still being held by the soil compared to the soil weight when dry. I find for deep containers, this method is a bit more accurate then simply putting your finger in as it takes the whole soil-mass into consideration.
Hi!
What do you think about soil´s moisture meters?
I have not tested them. I just don,t see a need for them-use your finger.
“Water when soil needs it – not on a schedule.” The challenge is that many gardens are on schedules imposed by regional watering restrictions – minimal water every second day in my area.
Certainly there are some who have the time to hand-water in between designated irrigation hours but it can be a full time job.
I am not talking about hand watering, which is mostly a waste of time. The key is to turn on the irrigation system when it is required.
I do understand the wisdom of your advice to water when plants need it . . . my point is that many cannot because of watering restrictions. Where I live, watering is limited to 2 hours every second day. I do hand water when I see plants suffering; not ideal but what’s the alternative?
Plant things that do not need to be watered.
“Irrigation systems are better for watering” – another myth? I have installed an irrigation system in the vegetable garden, including a timer and valves so that individual beds can be disconnected from the system. The system can be useful in the summer during extended dry periods or when I am on vacation. But I also override it during rainy periods. I have always wondered about the effectiveness of such systems for small gardens compared to overhead watering by rain or sprinkler. Sprinklers are often described as wasteful and inefficient, while irrigation systems like drip systems supposedly deliver water efficiently directly to the soil/plants. But is this another myth?
Overhead watering is not nearly as wasteful as claimed. Drip irrigation is also wasteful if not properly installed. Myth 152, from Garden Myths Book 2.
Interesting post 🙂 Questions:
1) Why would wind increase the evaporation rate?
2) Why would plants that are located close together remove water faster than if they were planted farther apart?
I appreciate your posts 🙂
1) In still air, the water vapor collects right above the surface of the ground. As the % of water in this layer goes up, evaporation slows slows down. If wind blows the water vapor away, the evaporation rate stays high. If humidity is 100%, wind has no effect.
2) More roots close together, so more water used up by the plants. Think of one tree per acre vs 100 trees per acre.
Your response was also interesting 🙂 Allow me please, to follow up on the one about density of roots. Don’t more roots in the ground also hold more water, and harbor more life in the soil? All that life stores water, no? Are these 2 factors I’m asking about trivial when it comes to storing water in the soil? Thanks a ton 🙂
I think the amount of soil relative to plant roots and microbes is probably much larger – I am guessing here. If that is true than the plants play a smaller role in holding the water.