Water More Efficiently In The Greenhouse

Good quality water is becoming scarcer and more expensive. So naturally, greenhouse growers are looking for ways to reduce their water bills.

More and more new products are coming to the market claiming to save water. However, growing media is still the most important factor in how successful a grower is when watering. A proper growing media actually helps growers save water.

One way to save water is to make the growing media capture more of the water you are supplying. In this space, I’ll discuss how various growing media materials hold and release the captured water and how those water-holding and release characteristics affect your watering practices and, in turn, plant growth.

Sales brochures on growing media often present the water-holding characteristic of a media as that media’s capacity to hold water so many times (e.g. eight to nine times) its weight. But you fill pots by volume. So you want to know how much water that media holds, say, in a gallon pot.

The best growing media companies give their media’s container capacity value, which tells the maximum amount of water a media can hold in a particular container size. However, the container capacity value still doesn’t show how the held water is released and how much of that water is actually available for plants.

So first, I’d like to discuss some basic principles of plant physiology and soil physics regarding water. Then, I will apply those principles to help you water most efficiently. This knowledge will help you understand plant responses to diverse growing media materials.

Sponge-Worthy Model

Plants take up water from growing media in a way that’s similar to how we remove water from a soaked sponge. When squeezing the sponge, we apply only slight pressure to remove water that can come off easily. We apply more pressure and squeeze harder to remove more water and so on. At some point, even with a lot of pressure, the sponge doesn’t release any more water, even though it still has some water in it.

In a similar way, a plant extracts water from growing media by generating suction pressure. As water content in the growing media declines, the plant generates more and more suction pressure to extract water. Generation of more and more pressure, of course, requires energy that plants could have spent elsewhere – to expand stems, leaves or flowers.

At some point, the plant cannot generate enough pressure to extract water from the growing media. Then the plant wilts, even though there is still some water in the growing media.

Pressure On Growing Media

Scientists measured pressures at which water is easily available or unavailable to plants in growing media. These are the suction pressures that plants have to exert to get water from growing media. This pressure is expressed here in “kPa” (or kiloPascal, a unit that is similar to “psi,” or pounds per square inch; 1 psi = 7 kPa).

Immediately after you water a growing media to saturation, water in that media is basically at zero pressure, and hardly any suction pressure is needed to extract that water. Soon, after drainage from the pot stops, the pressure needed to extract water in the growing media is about 0.3 to 1.0 kPa. Water held at such pressure is easily available to plants.

Water in a growing media that can come out until you exert 5 kPa is easily available to plants, meaning there won’t be much setback to plant growth in this pressure range. When plants are growing vigorously, water in growing media is generally at about 3.5 kPa.

When water in a growing media reaches a point where you have to exert a pressure beyond 5 kPa – and up to 10 kPa – water is still available to plants. But plants have to spend more energy to get that water.

When water in a growing media reaches a point where you have to exert pressures beyond 10 kPa, most plants start experiencing water stress. At these pressures, a growing media may still look or feel damp, and plants may not be wilting. But water therein is not easily available to plants, and growth reductions are already occurring.

Normally, a watering should have occurred by the time water in growing media reaches 7 kPa pressure. But, if growing media keeps drying beyond 10 kPa before re-watering occurs, plants slow their growth expansion. As a result, plants would have short stems and produce fewer and smaller flowers. Conversely, without water stress, you can get long stem roses, for instance.

There are definitive differences between field soil and soilless growing media in how they each supply water to plants. The amount of water available when a plant is exerting 5 to 10 kPa to draw water shows the buffer capacity of a growing media. This capacity ensures protection in case of a danger – like if someone forgets to water, if a heat wave occurs or during shipping to stores. Compared to water in soil, water in soilless media is held loosely. Soilless media release more water at lower pressures, so it is easier for plants to take up water in soilless media. This is a big benefit for plants growing in soilless media, as they need to exert only a little energy to extract water and use the saved energy for growth expansion.

But, there is a cost to the grower who now has to pay greater attention to the watering practices. Because there is only a limited volume of growing media in a pot, a plant can deplete water in that limited volume rapidly.

Growth Before Wilting

A wilting plant is a symptom we associate with water shortage. That’s because we can see that symptom. But well before a plant wilts, many plant physiological processes are affected and plant growth is damaged when water shortage starts to occur.

For a plant, water is not available equally over any pressure range, even over the so-called easily-available water range of 1 to 5 kPa. As water available to a plant starts to decline, it starts to conserve water by shutting its stomata through which water transpires (and cools the plant). But carbon dioxide used in photosynthesis enters the plant through the same stomata. Shut stomata means reduced photosynthesis, therefore reduced plant growth.

Although there are differences between plant species as to how much growth is reduced, growth reduction occurs even in cactus when there is shortage of water. Unhindered growth of a plant, therefore, depends on keeping a plant’s stomata open and its transpiration uninterrupted.

Transpiration rate of a plant, however, is not the same all the time. On a cloudy, cool, humid, still day, plants hardly transpire. On a sunny, hot, dry, windy day, transpiration can exceed the water available from growing media.

Takeaways

Your task is to ensure that water available to a plant synchronizes with the transpiration demand of that plant at any given time. And this means the growing media at any given time should have enough water available at least pressures, so the plant can exert the least effort to obtain water and reach its full potential growth.

Such a condition should exist for a long time so you don’t have to water often. And how can you achieve that condition? You’ll learn that in part two of this series.

Leave a Reply

More From Plant Culture...
Hendriks-Half-Open-Roof_GGS

March 26, 2015

10 Greenhouse Products For First-Rate Growing Environments

From coverings to fork-lifts, greenhouse suppliers offer a variety of products to make growing easier. Check out the slideshow to learn more about these, plus several other products that can offer you value, versatility and durability.

Read More
Rose rosette on Knockout rose, April 2012. Photo credit: Alan Windham, University of Tennessee

March 25, 2015

$58 Million In APHIS Farm Bill Funding Will Support Horticulture Priorities

Nearly $58 million as been allocated by the USDA's Animal and Plant Health Inspection Service (APHIS) to support the industry's Plant Pest and Disease Management and Disaster Prevention Program, under Farm Bill Section 10007. The program will support mitigation efforts for specialty crops, including providing research and other funding to address plant pest and disease priorities for the specialty crop industry, including floriculture and nursery crops.

Read More
AFE scholarship_Ryan Dickson

March 25, 2015

AFE Educational Grant And Scholarship Application Deadlines Approaching

Apply now for American Floral Endowment (AFE) scholarships or educational grants. Applications can be found online. For educational grants for 2015-2016, applications must be submitted no later than June 1. Scholarship applications are due May 1. AFE will award $40,000 in scholarships for 2015.

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
AmericanHort

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