How To Water More Efficiently

Clay curve

In Greenhouse Grower’s last issue, we discussed concepts of physiology of plant water uptake. In this article, we’ll discuss physics concepts pertaining to growing media’s capacity to store and release water. Then, we’ll apply the concepts to improve your watering practices.

Pores In Growing Media

Different growing media have different capacities to store and supply water to plants. These differences are due to pore spaces in growing media.

Water obviously requires a space to stay. In a growing media, water is held in pore spaces. When growing media materials are mixed, pores form between their particles. Large particles form large pores and small particles form small pores. However, in a mixture of large and small particles, small particles occupy and fill the large pores between large particles.

Water in large pores drains first and fast. Water comes out of such pores with little pressure. Small pores, on the other hand, hold water firmly. Thus, as the size of pore space decreases, water is held more tightly. An increasing pressure is needed to pull water from smaller and smaller pores. So, if a growing media has very small pores, although it holds more water, less of that water is easily available to plants (for example, water in clay).

Although the pore spaces between particles influence how much water is held by a growing media, there are other influencing factors. Pores inside a particle also influence the water held. For instance, we can test sand and peat with the same sized particles, which would form the same pore sizes between their particles. But peat holds more water because peat particles are open and have pores even within them.

Some growing media components like peat and coir have an exceptionally high level of internal pores that are also open, interconnected and accessible to water. That’s one reason these components have high water-holding capacity. If some of these internal pores are very narrow, water therein requires more pressure to come out.

In perlite and vermiculite, some internal pores are disconnected and closed. In expanded clay pellets, all internal pores are closed. Such internal pores are not effective in holding water.

Water is also held by solid particles as a coating or film on their surfaces. The affinity with which such water is held differs between materials used as growing media. If a material’s affinity for water is stronger, more pressure is required to extract such water.

All these factors influence how a growing media releases water to the plants under different suction pressures.

Curves Of Growing Media

How a growing media releases water at different pressures can be determined and drawn as a curve. For this, a saturated media is gradually de-saturated by applying either pressure or suction. As the media dries, its water content is determined. Such determined water release curve is unique property of that growing media. From this curve, one can draw conclusions on the water release abilities of that growing media and then apply proper watering practices.

Water release curves of some often-used growing media materials are presented in the chart at right. Look at the curve for peat. Soon after watering and free drainage, peat would have about 80 percent water. If you multiply this percentage by the pot size, say a gallon pot, you get 0.8 gallons or about 100 ounces as the amount of water in a gallon pot. By 5 kilopascals (kPa) pressure, the water amount is about 0.4 gallons or 50 ounces. So, 50 ounces of water is easily available to a plant in a gallon pot. If you don’t water even until 10 kPa pressure, about 10 more ounces of water is still available to that plant.

Now, look at the rockwool curve. Immediately after watering, rockwool holds as much water as peat. But rockwool releases almost all the water with very little pressure. Virtually no water is retained by 5 kPa pressure and, thus, rockwool has no water buffer capacity. Water stress can set in on plants growing in rockwool if a watering is not scheduled ahead of 3 kPa pressure.

In rockwool and other fibrous materials, water is held in large pores at contact points between the particles. Major portions of such water drain simply by the pressure induced from gravity. Major portions are also lost to air – often before plants can hardly use it. Because little water is retained in such materials for later use by plants, these materials require frequent watering.

Now, look at the curve for sand. Even if we apply a lot of water, sand can’t hold as much water as peat. And much of that water is also released at very low suction pressures. So, for sand too, you have to come back and water frequently. Also, because of the small pores in it, sand doesn’t release all the water and retains a certain amount of water, even with increasing suction pressure.

Hydrating Growing Media

One can grow in many materials but how one waters these materials should differ. As we saw, materials with substantially different water release curves require substantially different watering schedules.

You can schedule efficient watering electronically. There are sensors that can be placed in the growing media to know what pressure water is at held there. Then, you can automate watering – for instance, initiate watering at 7 kPa and terminate at 1 kPa.

You can also schedule efficient watering manually if you base it on the calculations from the water release curve of your growing media. Suppose your hydrangea growing in a gallon pot requires (including for evaporation) 12 ounces of water per day. As we’ve seen in the examples already shared, plant useful water reservoir size is about 60 ounces in peat and about 6 ounces in sand. So, in peat, you would have five days interval before you re-water, whereas in sand you would have to water twice a day.

On the other hand, instead of fast, vigorous growth, if you can accept a slow growth rate, you can supply less water to your plants. In fact, you can use water as a tool to control plant growth and size. But, be careful not to lose plant and flower quality in the process. Water-stressed plants would be short with fewer branches and fewer flowers. However, good marketing might call the stressed plants hardened or toned!

You also have to consider the shelf life of plants at retail stores because of the watering practices there, which can be infrequent and erratic. If water available to plants is less than their transpiration demand, plants stress. They appear dull and gray. They also shed leaves, abort flowers and, of course, wilt permanently in response to the water stress. If this happens, you would lose sales.


As you can see, an efficient balance of obtaining the desired plant growth and saving water is required. But now that you have armed yourself with the data on capacities of various growing media materials to store and release water, you have a better ability to decide which option is best in your situation to obtain the desired plant growth and save water.

Leave a Reply

One comment on “How To Water More Efficiently

  1. In your article above, you mention a chart at the right, in the Growing Curves portion of your article. Am I missing it somewhere? I do not seem to be able to find the chart you are referring to in your article. Can you help me to locate it? I really like your article, it's very informative, not to mention timely. Thank you, Nadine

More From Plant Culture...
Status of Marijuana US Map May 2015

May 29, 2015

Marijuana Legalization Updates

As of April 2015, 23 states and the District of Columbia (D.C.) currently have laws legalizing marijuana in some capacity. At the federal level, several bills are currently awaiting action. Here is an update on current state and federal marijuana legislation across the U.S.

Read More
cannabis, marijuana plant

May 29, 2015

Making The Decision To Produce Medical Marijuana

Should horticultural growers consider cultivating medical marijuana? That’s up to the individual grower, of course, and certainly a number of growers already have jumped in. We at Meister Media Worldwide, publishers of Greenhouse Grower and American Vegetable Grower, do not necessarily endorse nor oppose the production of medical marijuana. But we do feel it is an option worth exploring. We intend our “Medical Marijuana” series of articles to give you information you need to make your own call. We hope you find it useful, and we do welcome your comments, thoughts and ideas as we continue to cover what we’re fairly certain is only going to be an increasingly viable and growing market for this emerging crop.

Read More
Double Whammy Merchandise Display At CAST2015

May 28, 2015

Grow Inspiration To Grow The Horticulture Industry

We need to work together as the horticulture industry to inspire and instruct consumers with our plant knowledge and marketing expertise.

Read More
Latest Stories
Bee on a Sedum

March 17, 2015

4 Key Pollinator Research Projects To Be Funded By Hort…

The Horticultural Research Institute will grant $125,000 in financial support for four key projects as part of the Horticultural Industry Bee & Pollinator Stewardship Initiative. The Initiative has three primary goals. First, to convene a task force to develop a bee and pollinator stewardship program, including creation of best management practices for plant production. Second, to identify and fund research that will help answer key science questions and fill gaps needed to design and refine the stewardship program. Third, to seek to positively position the horticultural community and its customers by collaborating with other compatible groups interested in augmenting pollinator habitat and protection.

Read More

March 11, 2015

Pollinator Initiative Promotes Bee-Friendly Talking Poi…

AmericanHort and the Society of American Florists are working tirelessly with the ornamental industry's Pollinator Stewardship Initiative on a number of new projects.

Read More

February 11, 2015

Infusion Technology Boosts Seed Performance, Study Sugg…

Seven-year-old wheat seed germination can increase by as much as 83 percent, according to a Vital Force Technology Study that looks at the effects of energy infusion technology on plant vitality.

Read More

February 3, 2015

American Floral Endowment Accepting Research Pre-Propos…

If you are pursuing a floriculture research project, now is the time to apply for funding through the American Floral Endowment. Research pre-proposal applications for 2015-2016 funding are due to AFE by June 1, 2015.

Read More

January 27, 2015

Marijuana’s Trajectory And Ascent To Horticultural Cr…

Marijuana growing is poised for change as growers and researchers focus on improving production practices.

Read More

December 9, 2014

Greenhouse Production: Two Years Of Basics & Beyond…

Greenhouse Grower's Basics & Beyond articles cover some of the latest news and research going on in greenhouse production. Here are article links for the last two years.

Read More
GrowIt! App Wins Gold At Design100 2014 US Mobile & App Design Awards

November 24, 2014

GrowIt! App Wins Gold At Design100 2014 US Mobile &…

The social garden app GrowIt! takes the Gold Winner award at the design100 2014 Mobile & App Design Awards.

Read More

November 10, 2014

The Perennial Farm Joins HGTV HOME Plant Collection

The Perennial Farm joins the HGTV HOME Plant Collection growers' network for 2015.

Read More

November 4, 2014

AmericanHort Publishes Revised American Standard For Nu…

AmericanHort announces the revised American Standard for Nursery Stock (ANSI Z60.1) is now available for industry use. The Standard reflects the consensus of the industry regarding how nursery stock — living plants other than annuals — should be specified and sold within the trade.

Read More

September 26, 2014

Master The Art Of Watering

Watering is elemental to healthy plants, but one of the hardest concepts for new employees to master in the greenhouse. Recommend these tips to start them off right.

Read More

September 16, 2014

Ball FloraPlant’s Las Limas Facility Provides Gro…

Ball FloraPlant’s Las Limas farm in Esteli, Nicaragua, is one year away from full production, but sales and quality from the two-year-old facility are right on track.

Read More
Erysimum 'Cheers' from Darwin Perennials

September 15, 2014

Darwin Perennials Takes Production Offshore In Bogota, …

With its recent purchase of a farm in Colombia, Darwin Perennials is ready to amp up supply of its perennial genetics, to provide growers with tried-and-true varieties and comprehensive production specifications.

Read More

July 23, 2014

Plan Now To Prevent Bract Edge Burn On Poinsettias

Reduce fertilizer and water, and allow your poinsettias to develop slowly during the final four weeks of production to avoid bract edge burn.

Read More

July 11, 2014

Growing Your Crops Above Their Base Temperature

Lowering temperature set points in the greenhouse may help you combat rising heating costs.

Read More

May 1, 2014

Growers Report Nutritional Problems On Geraniums

In recent weeks, several growers have contacted Michigan State University Specialists about leaf discoloration on geraniums, especially the purpling of lower leaves.

Read More

April 22, 2014

How Two Postharvest Care Products Worked On Potted Plan…

What your potted plants look like at retail translates to sales or fails. North Carolina State University researchers report on how two postharvest care products performed.

Read More
Dianthus 'Passion' from Emerald Coast Growers

March 27, 2014

Growing Dianthus Successfully

Here's some advice on transplanting and producing this classic perennial favorite.

Read More
Aquilegia canadensis

March 10, 2014

Tips For Producing Aquilegia

Advice on planting, temperature, vernalization, lighting and more on columbine from Emerald Coast Growers' head grower Josiah Raymer.

Read More