THE GREEN THUMB ALMANAC 

Your Online Gardening Information Center

Watering

On This Page!

  • The Science - Understanding Water
  • The Art - Knowing When to Water
  • Watering Newly Planted Trees & Shrubs


Knowing when and how much to water is as much an art as a science.  Learning the science of watering is relatively easy...it is the art of watering that is most difficult.  Why?  Because the art of watering involves experience and experience takes time.  This page is the most important page on this entire website.  If you master both the art and science of watering you will be able to grow anything!  Remember, improper watering is the #1 killer of plants!

The Science - Understanding Water

The science of water involves how water behaves in different types of soil conditions.
  • Water wants to stay together, this is called cohesion.  Water also follows the path of least resistance.  This can be observed when you try to water soil that has become too dry.  Dry potting soil, for instance, will actually repel water.  In a container this means that the dry soil will shrink and pull away from the sides of the pot.  Then, when water is applied, it will run off and down the sides of the pot and out of the drainage hole leaving the plants root ball dry.  In this instance you will have to double- or even triple-water the container before the soil is moist enough to absorb water.
  • Another principle of water is called capillary action.  This is the movement of water upwards from the bottom of the pot.  So when you sit a dry pot in a saucer of water it sucks the water upwards.  This is a good thing if the plant needs water.  If the plant is already moist, however, it can be a negative by keeping the root ball saturated.  So if the soil stops absorbing water after an hour, empty the plant saucer of any remaining water.  Most plants do not like to sit in water for an extended period of time.
 
  • Plants absorb water through the process of osmosis.  Osmosis is the movement of water from an area of high concentration to an area of low concentration.  So when a plants roots are drier than the surrounding soil the water moves through the roots cell walls and into the plant.  At the same time nutrients, such as fertilizer, that have been dissolved in the water also move into the plant.
  • Plant roots also breath!  So, in addition to water and nutrients the roots are absorbing oxygen.  This is a good thing to remember when buying potting soil.  A good potting mix will be fluffy and have lots of air spaces.  Soil that is too heavy does not have enough air spaces.  Also, soil that is too wet has all of the air spaces filled with water.  This keeps the plant from being able to absorb oxygen.
There are many other factors that will determine when you need to water.
Wind, humidity, and temperature will all affect how quickly soil dries out and how quickly a plant will use water.

The Art - Knowing When to Water

So, we have a general idea of the science of water but how do you know when to water?  This is the art part of watering.  Experience will be your best guide.  Why?  Because it is only by killing a plant a few times that we learn how to grow it.  I know this sounds cruel, but it is true.  As you become a better gardener you will learn more about plants, where they come from and how frequently they like to be watered.
  • Knowing a plants natural environment is very helpful.  We all know that cacti and succulents come from dry, arid regions and that many tropical plants come from wet rain forests.  This kind of knowledge gives you a hint about how often a plant would like to be watered.  
  • Get a feel for soil...literally.  The best way to determine if your plant needs water is to observe the plant and feel the soil.  Does the plant look unhappy or wilted?  Feel the soil.  If the soil feels moist and the plant is wilted it has probably been over watered.  If the soil feels dry and the plant is wilted, it probably needs water.  If you are unsure, purchase a moisture meter available at most garden centers.  When placed into the soil the meter will tell you how moist the soil is.
  • Check the root system.  Don't be afraid to pull a plant out of its pot.  See how moist the soil is down deep in the pot.  It won't hurt the plant!  Another way to see if a plant needs water is to feel the weight of the pot.  Water the plant and get a feel for how heavy the pot is when well watered.  When the pot feels light in weight then that means the soil has dried out and your plant may need water.
  • How frequently should you water?  It is not the amount of water given to a plant but rather the frequency of watering that kills most plants.  Always water deeply but infrequently.  I know it is a temptation to water your plants once a week when you clean house but this is a bad practice.  It would be good to check the soil in your pots when cleaning but only water if the soil feels dry.  Plants will actually use water at different rates during the year so only water if they need it!
When watering a container you must apply an equal volume of water to the volume of soil in the pot.  For instance, a pot which contains 1 gallon of soil should have at least 1 gallon of water applied.  Don't give a large plant a small cup of water!
 
Water the soil...not the plant!
Wet foliage can encourage disease.

Watering Quick Tips

  • Water deeply & infrequently.  Watering briefly each day will not apply enough water to reach the roots of your plants.  Water long enough to moisten the soil 5 or 6 inches deep where the roots are.
  • Water early in the day.  Water evaporation is greatest in the middle of the day when the sun is highest.
  • Mulch.  Put down a 3 inch layer of mulch to decrease water evaporation and lower soil temperature.
  • Weed your garden.  Weeds compete with your plants for available moisture.
  • Place a bucket out with your sprinkler.  Most lawns need at least 1 inch of water a week to stay green.  When your bucket has 1 inch of water in it, you are done!

Watering Newly Planted Trees and Shrubs

Watering plants properly from day 1 is the best way to increase their chances of surviving past the first growing season.
Here are some tips on how to do just that.
1.  Create a berm just outside of the planting hole that will retain water long enough to soak the root ball of the plant (often times the back fill soil will saturate much more quickly than the root ball itself, leaving the root ball relatively dry).
2.  Heavily water your tree or shrub prior to, immediately after, and the day after planting date.  This insures all voids between the root ball and ground will be filled.  Having too much air contacting the roots can stress plants.
3.  Maintain a watering schedule.  Water twice a week (three times a week in +95 degree weather) for the first month (when there is less than an inch of rain per week).  Ideally a 5 gallon bucket with a few holes poked in the bottom and sides should be sufficient for large trees and shrubs.  The slow release will allow for the water to soak in before it becomes runoff travelling away from the root ball.
4.  After the first month, 10 gallons (add 10 gallons per inch of diameter for trees greater than 2") once a week will percolate deeper into the soil, coaxing a deeper root system.  Smaller trees may require less amounts of water, but be sure the root ball is thoroughly saturated.  During excessively hot weather (95+), go ahead and water twice a week as the soil begins to dry out much more quickly.
5.  Don't neglect your plants during the winter!  Just because they drop their leaves doesn't mean they don't get thirsty.  During winter months, however, watering amounts should be reduced by half as long as the ground isn't frozen.  Shorter, cooler days make plants require less moisture.  Make sure the ground isn't frozen, and a mild temperature is forecasted for the next 2-3 days that won't freeze the ground deep enough to harm the roots.  Winter rules especially apply to evergreens.  Because evergreens maintain their leaves, they lose more water moisture than deciduous trees and shrubs.
6.  During the second and third growing seasons after planting, continue to water trees and shrubs every 10-14 days if it doesn't rain and soil moisture indicates the need to water.  As the root system of these plants extends during establishment, water in a wider ring around the plants, soaking the soil to a depth of 8 to 12 inches.
7.  Turf grass growing over the root system of young trees competes for moisture.  To reduce competition, maintain a grass-free area around the base of the young tree, extending to the drip line of the outer branches or beyond.
8.  An organic mulch around the base of young trees and shrubs is recommended to keep the soil moisture more uniform and to stabilize soil temperature.  Apply and maintain an organic mulch ring 2 to 3 inches deep around the base of the tree, covering the grass-free area.
(Credit: Emily Nolting et. al., Watering Newly Planted and Young Trees and Shrubs, Kansas State University, January 2008.)