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...

July 1, 2015

Rough Brothers Is Acquired By Gibraltar Industries

RBI Solar Inc., Rough Brothers Manufacturing Inc. and affiliates have been acquired by Gibraltar Industries, a manufacturer and distributor of products for residential and industrial markets, for $130 million. Capitalizing on its 80 years of design-build experience and leadership as the largest greenhouse manufacturer in North America, Ohio-based RBI has established itself during the past five years as North America’s fastest-growing provider of photovoltaic (PV) solar racking solutions. The transaction will enable Gibraltar to leverage its expertise in structural metals manufacturing and materials sourcing to meet global demand for solar racking solutions. The company also announced that it anticipates its second-quarter 2015 financial results will be in line with its previous expectations. “Acquiring RBI is an important step in the transformation of Gibraltar into a company with a higher rate of growth and best-in-class financial metrics,” says Gibraltar CEO Frank Heard. “This acquisition directly aligns with key end markets and product platforms […]

Read More

June 30, 2015

Opportunities Abound For Women In Horticulture, Says Dosatron International’s Lela Kelly

My love for horticulture goes back to my grandparents who were farmers in upstate New York. My family’s involvement in agriculture left a lasting impression that has spanned decades, and still drives my passion for our industry today. I started my career in horticulture when I owned a greenhouse, nursery and garden center in Sayville, Long Island, N.Y. We specialized in ground cover production, later branching out to producing perennials and bedding plants.     I then went onto wholesale hard goods distribution. In the 1980s, I was the first woman salesperson on Long Island. What an eye opener that was! New York is a very special place, different from anywhere else in the country. You cannot imagine how difficult it was walking into greenhouses and nurseries as a woman, in a man’s world. Luckily, I grew up in New York City in an Italian family, who taught me great […]

Read More
Medal of Excellence Award

June 30, 2015

Get To Know The 9 Variety Finalists For Editor’s Choice And Industry’s Choice Awards

Greenhouse Grower‘s Evening Of Excellence reception is rapidly approaching. Here, you can learn more about the varieties that are finalists for the Medal Of Excellence Editor’s Choice and Industry’s Choice Breeding Awards. Thank you to the 2015 sponsors of Greenhouse Grower‘s Medal Of Excellence Awards, Landmark Plastic, Nufarm and Stockosorb. Editor’s Choice Asclepias ‘Monarch Promise’ (Hort Couture) Basil ‘Balsamic Blooms’ (EuroAmerican Propagators) Celosia ‘Dragon’s Breath’ (Sakata Seed) Dianthus Jolt Series (PanAmerican Seed) Salvia Grandstand Series (Green Fuse Botanicals) Scabiosa ‘Kudos Pink’ (Hishtil Nurseries) Industry’s Choice Basil ‘Balsamic Blooms’ (EuroAmerican Propagators) Bidens BeeDance Series (Suntory Flowers) Celosia ‘Dragon’s Breath’ (Sakata Seed) Dianthus Jolt Series (PanAmerican Seed) Lobelia ‘Starship Deep Rose’ (Kieft Seed) Vinca Valiant Series (PanAmerican Seed)     Join us Monday, July 13 in Ballroom 2 at the Columbus Convention Center to find out which varieties will receive the coveted awards. The reception begins at 5:15 p.m. and the ceremony […]

Read More
Latest Stories
cannabis, marijuana plant

June 27, 2015

Concern Grows Over Unregulated Pesticide Use On Cannabi…

As most growers know well, the federal government regulates all insecticides, fungicides, herbicides and other commercial chemicals used on agricultural crops. Therein lies the problem with use of chemicals on cannabis crops – so far, the feds want nothing to do with legalized marijuana. According to “Concern Grows Over Unregulated Pesticide Use On Cannabis,” a June 17 article on the National Public Radio (NPR) network by Agribusiness Reporter Luke Runyon, the lack of regulated chemicals for cannabis has left growers to experiment on their own. “In the absence of any direction the subject of pesticide use on the crop has just devolved to whatever people think is working or they think is appropriate,” said Colorado State University Entomologist Whitney Cranshaw in the NPR report. “Sometimes they’ve used some things that are appropriate, sometimes unsafe.” Denver officials held tens of thousands of marijuana plants earlier this year due to safety concerns, but […]

Read More

June 16, 2015

The Butterfly Effect: Insect’s Wings Key To Azalea Poll…

A researcher from North Carolina State University (NC State) has found that in the case of the flame azalea (Rhododendron calendulaceum), all pollinators are not created equal. In fact, due to the flower’s unique reproductive structure, butterflies — and specifically, their wings — are the key to pollination. The flame azalea is commonly found in the Appalachian Mountains, ranging from as far north as New York to Georgia in the south. Like most azaleas, the flowers are large, and have an unusual structure: both the anther (male) and stigma (female) parts are very elongated and separated from one another. NC State biologist Mary Jane Epps was interested in how the azalea’s flower structure affected its pollination. “In order for a plant to reproduce, a pollinator — usually an insect — has to spread the pollen from the anther to the stigma,” Epps says. “In the case of the flame azalea, […]

Read More
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