As with nearly every other factor, landscape plants can vary widely in their requirements for water. Knowing the needs of your particular plants is important so that you apply water appropriately. While most plants come under the standard one inch of water per week throughout the growing season, some will be over-watered by this amount while others will be short of water.

Types of Plants

One of the first pieces of knowledge that the prospective home landscape irrigator needs to know is the amount of water a plant needs or will tolerate. This amount can vary quite a bit and the job is especially challenging in beds and borders where dozens of different species from all over the world are thrown together.

The following describes some broad categories that might help you to figure out how to irrigate your plants. These are based on plants growing in the soil. Those grown in containers may be of similar classifications but the growing media used may make all the difference so these are addressed in their own section.

  • Can't Tolerate Drought - These are often plants that originated from the margin of lakes or streams or other always moist areas. They are used to having a ready, plentiful supply of water available to them throughout the season.

  • "Average" Plants - The vast majority of landscape plants fall into this category. They need a slightly moist soil most of the time. They can withstand short periods of droughty conditions and even short stretches of wet soils. These are your "standard" plants that need about 1 inch of water per week in either rain or irrigation to do their best.

  • Can't Tolerate Wet Soils - Plants that developed on the plains or in or around desert areas fall into this group. Mediterranean plants, succulents, cacti, and others are from areas that receive very little rain per year. Too much moisture can lead to root rots.

Soil Types

Landscape soils are made up of three inorganic particles - clay, silt and sand. Clay particles are very small and fit together very tightly leaving almost no space for openings (pores) to hold water or air vital to plants. Sand particles are relatively much larger and allow for large pores that allow the rapid movement of water. Silt particles are in between the size of clay and sand.

The fourth part of most soils is organic matter or humus. This material adds to the ability of sandy soils to absorb water and forms clusters of clay particles to allow more space for drainage to occur. For more on soil composition...

Soil Water Holding Capacity

The soil is often compared to a sponge when it comes to explaining the process of soil water holding capacity. Here are the descriptive categories for soil moisture:

  • Saturation - When the sponge (or soil) are completely full of water and can hold no more, it is said to be saturated. Such soils have all of their pores filled with water and there is no oxygen space available. The roots of most landscape plants must have oxygen in order to grow and function properly. That is why clay soils that tend to be saturated after heavy rain are difficult environments for plants.

  •  Field Capacity - If you gently squeeze the sponge,  some but not all of the water will drain out of it. The water that is left is held by what is called capillary action. In the soil, this is the amount of water that is the best for the plants. There will be about 25% of the pore space filled with water and 25% of the pores will be full of air. Field capacity is the goal of all watering and irrigation of landscape plants.

  • Wilting Point - If you squeeze the sponge as hard as you can, almost all the water will be lost. When a soil reaches this level, only the water that is closely stuck to the soil particles are left. Plant roots generally cannot use this water so, they will begin to wilt.

Goals of Watering or Irrigation

Of course, the primary goal of applying water to our plants is to allow them to grow and thrive to the best of their ability by meeting their particular needs. There are two elements that contribute to meeting this goal:

  1. Knowing How Much to Apply - As discussed in the Types of Plants listing above, different plants have differing requirements for water during the growing season. Therefore, you need to know the plants that your are growing and what they need. Most plants need about an inch of water per week in either ran or irrigation so you can use that as a starting point. But, also be sure to find out which of your plants might be either drought resistant or drought intolerant.

    Remember too that the amount applied in your landscape will depend on your Soil Type. Depending on the relative levels of sand, silt, clay and organic matter in your soil, the amount and frequency of irrigation may vary. If the plants need one inch of water and you have heavy, clay soils, you may need to make two applications of 1/2 inch per week to avoid saturation. In sandy soils, perhaps two applications of 1 inch each per week will be needed to assure that one inch is actually available to the plants. In a good "loam" soil with a lot of organic matter, an application of one inch at one time during the week might suffice.

    Generally, the goal will be to put on enough water that it soaks down to the root zone of the plants and will stay there for enough time to be taken up by the plant. Grass and most perennials have roots that are clustered in the 4 to 6 or 8 inch depth. Trees usually have most of their roots in the top 18 to 24 inches of soil.

  2. Knowing When to Apply - For most plants, the soil needs to be moist but not water logged or bone dry for the roots to take up the necessary water. To meet this demand, you really cannot just water on a calendar schedule.

    We have already discussed how water needs may vary by the type of plant or the soil type in your garden. These will partially determine how fast the water moves out of the root zone of the plant. However, other factors such as the air temperature, soil temperature, wind velocity and humidity in the air will also be factors on determining when to apply water to your plants.

Watering Techniques

There are several techniques that can be used in the home landscape to deliver water to the root zone of your plants. In the end, it really doesn't matter which one you use as long as the water gets where you want it to go. So the decision boils down to such factors as personal choice, cost, availability, ease of use or any other factor.

Measuring the Amount Applied

There are several ways to measure how much water has been applied. One method involves a simple measurement by placing an open container of some sort under the sprinklers and measure the amount in it after a period of irrigation. Don't assume that your system will deliver the same amount of water in an hour as a friend's system. Your water pressure and types of hoses may make a difference in delivery rate.

Another way, especially for drip irrigations systems, is to run the irrigation for a certain amount of time and then dig down in the soil to the typical root depth of your plants. After all the whole goal is to get water to the roots so measuring the depth of water penetration over a period of time will tell the tale.

Note: We have provided some general information and observations on this topic aimed at the home gardener. Before you take any serious action in your landscape, check with your state's land grant university's Cooperative Extension Service for the most current, appropriate, localized recommendations.


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