While you are most likely familiar with traditional treatment technologies like mechanical filtration, chlorination and ozonation, you may not be as familiar with biological and ecological treatment technologies. Innovative ecological treatment methods such slow sand filters, bioreactors and algal turf scrubbers are low-cost, low-energy technologies that harness the power of naturally occurring microorganisms to remediate nutrients, agrichemicals and pathogens in water.

Additional benefits of using these technologies include no need for chemical input, no production of unwanted disinfection by-products, no need for technical instrumentation or specially trained personnel, low operating costs and minimal maintenance. Installation costs of these systems typically range from several hundred dollars up to around $10,000, depending upon the volume of water to be treated. These costs are substantially lower than installation, maintenance and labor costs of other disinfestation and remediation technologies.

Slow sand filters

European horticultural growers have been using slow sand filters (SSFs) since the early 1990s to remove plant pathogens from recycled irrigation water. However, the technology is actually much older than that — SSFs have been used since the early 1800s to clean contaminants from water for human consumption. Today, SSFs at nurseries are primarily used to remove plant pathogens, including Phytophthora, Pythium and Fusarium. However, recent research has also shown that slow filters containing substrates other than sand (such as pumice and rockwool) may be able to effectively remediate pesticides and even nutrients from irrigation water.

SSFs consist of three major components (Fig. 1):

• Sand (or other substrate, top layer) – facilitates the purification process, typically about 2 to 4½ feet deep
• Gravel (middle layer) – supports the sand bed and prevents sand from clogging the underdrainage
• Underdrainage (bottom layer) – Captures cleaned water and allows for movement of water through the system

The water reservoir is placed on top of the sand layer and should be maintained at a depth of about 3 to 5 feet to maintain the necessary water pressure to ensure consistent flow through the SSF. Recommended filtration rates range from about 150 to 350 gallons per day per square yard of surface area. So, if you have a 15-foot diameter tank installed, you may be able to treat up to about 7,000 gallons of water per day onsite. While faster flow rates may still achieve sufficient remediation, the filters would be more likely to clog and thus require more frequent maintenance. The ability of the filter to treat water at faster flow rates depends upon the quality of the water entering the filter. If the incoming water was previously held in a settling tank or pond, or has been pre-filtered to remove any organic solids or sediment, higher flow rates can be achieved. The quality of the sand is also critical for optimal treatment — sand should be pre-washed, and grains should be about 0.01 inch (0.3 mm) in diameter, or about 60 mesh.

How exactly does a bunch of sand and rock remove plant destroyers like Phytophthora? Well, we’ve got microorganisms to thank for that. Over the course of about three to four weeks, a biologically active film layer naturally forms in the top few inches of the SSF. These microorganisms help to trap and degrade pathogen contaminants. Periodic maintenance, including the removal of the first few inches of sand, is required to maintain the designed flow rates through the SSF. Installation of two SSFs in parallel is recommended for horticultural applications — this way, while one SSF is shut down for maintenance, the other can remain in operation. However, remember to start cycling water through the alternate filter about a month in advance of use to allow that crucial microbial layer to form at the surface of the SSF.

Bioreactors

Agricultural denitrifying bioreactors, sometimes called carbon beds or carbon walls, are lined trenches or containers filled with a carbon-rich material, such as woodchips or woody substrate. Bioreactors are commonly used to remediate nitrate-rich irrigation runoff and have been applied extensively in agricultural production around the world, including in corn and soybean fields of the Midwest. Bioreactors installed at the edge of row crop fields of the Midwest have been reported to be as large as 120 feet long and 25 feet wide and treat drainage from up to about 80 acres of production area. Sizing of the bioreactor is dependent upon size of production area, estimated irrigation runoff/drainage amount and nutrient load. Experimental bioreactors have been installed at nurseries and greenhouses in the U.S. These horticultural bioreactors are typically smaller than those currently being used in row crop agriculture — on the order of just a few cubic yards and can handle high runoff flow rates without flooding production areas. While the longevity of woody substrate in bioreactors is still being researched, current estimates range from around 10 to 20 years. Both hardwood and softwood substrates have been used in bioreactors.

The typical bioreactor layout (Fig. 2) allows for irrigation drainage water to enter the bioreactor on one side, flow horizontally through the bioreactor via a series of perforated pipe and exit the bioreactor at the opposite end. Naturally occurring microorganisms populate the carbon-rich woody material and reduce nitrate as part of their everyday routine. Flow control structures — one at the bioreactor entrance to divert water and another at the exit to control water height — may be used. Stop logs in the control structures may be raised or lowered to control flow rate. Bioreactors may also be designed to allow for bypass flow during peak flow conditions.

While bioreactors are extremely effective at removing nitrate from water (with removal efficiencies up to 100%), unintended negative consequences of bioreactor use may include the release of greenhouse gases. These side effects are currently being investigated and may be alleviated by maintaining proper flow conditions through the bioreactors. Recent research has revealed that in addition to nitrogen andphosphorus, some types of bacteria and herbicides may also be remediated by bioreactors.

Algal turf scrubbers

Algal turf scrubber (ATS) systems consist of screens covered with attached algae (Fig. 3). Nutrient-rich water is typically pumped over these screens in a shallow trough, or even in open water. The attached algae use the power of the sun to uptake nutrients like nitrogen and phosphorus. Because algae grow so quickly, this technology removes nutrients at a very high rate. It is important to remove (scrape off) the algae from the screens about once a week, as this will rejuvenate the algal community and promote high growth rates. The harvested algae is incredibly rich in nutrients, and thus can be used as a substrate amendment, fertilizer or biofuel feedstock. These ATS systems have been shown to remove almost a pound of nitrogen per 10 square yards per day. Optimization of ATS systems depend on several factors, including flow rate, flow type (pulsed vs. continuous) and pH. Research on how these factors influence efficiency of ATS systems is currently being conducted.

Combination treatments

While many single traditional treatment technologies, as well as the ecological technologies described above, can effectively remediate common agricultural contaminants from water, the combination of several of these technologies in series or “treatment trains” may result in the most efficient contaminant removal, aka the most bang for your buck. Unfortunately, there is no single treatment system out there that will effectively manage all types of contaminants. The types of technologies that should be used in combination treatment trains depends on site conditions, the types of contaminants you’re interested in targeting and your budget. The use of treatment trains for water treatment technologies in container-grown crop production is an emerging topic of research, as little work has been done regarding treatment train efficacy and order of technology application for targeted contaminant removal in agricultural production systems.

Reliable access to fresh water is an increasing concern for many growers, as water supplies continue to decline due to overuse and increased incidence of drought. Additionally, more regulations on water withdrawals, water-use efficiency and quality of irrigation runoff water entering surface waters are being implemented. Therefore, more and more growers are looking into ways to remediate and reuse their irrigation runoff water onsite. The ecological treatment methods discussed in this article are not only low-cost and low-energy but are also safe and easy to use. Implementation of slow sand filters, bioreactors and ATS systems expand the tools available to the green industry to facilitate more efficient and sustainable water use and management. Effective remediation of contaminants, low costs and environmental stewardship do not have to be mutually exclusive. Consider adopting new and nontraditional ways to manage water to meet your irrigation water demands.

Natasha Bell (bellna19@ecu.edu) is an assistant professor at East Carolina University’s Department of Engineering and was a Graduate Research Assistant in the Department of Plant & Environmental Sciences, at Clemson University, Clemson, S.C., when this article was originally published. Dr. John Majsztrik (jmajszt@clemson.edu) is a research assistant professor and Dr. Sarah White (swhite4@clemson.edu) is a professor in the Department of Plant & Environmental Sciences at Clemson University, Clemson, S.C.

Acknowledgments: Funding for this material is based upon work that is supported by the National Institute of Food and Agriculture, U.S. Department of Agriculture, under award number 2014-51181-22372.

Editor’s note: This article originally appeared in sister publication Nursery Management.

Dr. Aaron Palmateer joined SePRO last month as the company’s new Technical Development Leader for Ornamentals. In his time working as a professor at the University of Florida and in the private sector, Palmateer has become one of the horticulture industry’s leading voices on plant pathology.

In joining SePRO, Palmateer says he’s excited to work directly with the company’s line of products and help growers apply them effectively. At the company, he’ll be working mostly with a portfolio that includes fungicides, herbicides, insecticides, and plant growth regulators, but also looking at developing aquatic products used in other industries for ornamental use.

“SePRO is really investing in and wanting to move forward in the greenhouse business,” Palmateer says. “Their current portfolio is really unique in a lot of ways. Just look at the insecticides — Rycar®, Hachi-Hachi® SC, Talus®, Akari® — they are all in unique insecticide resistance classes. They play well with others, so growers can really position SePRO products as part of a rotation.”

“There’s just a ton of outstanding chemistry,” he adds.

A healthy industry

In talking with growers and other industry members at Cultivate’21, Palmateer said one of his major takeaways is that plants are continuing to sell quickly and often in big numbers. This trend began in 2020, but at least for now, it appears to be continuing.

“Plants are flying off the shelf quickly,” he says. “There’s less need for longevity of products and some growers are seeing less pest pressure because they aren’t having to hold plants as long. We’ve actually seen less reliance and need [for some products right now] because of how fast things are moving. I’ve heard that from a number of growers.”

Palmateer also notes that growers are selling product that, in a normal year, they might not have been able to sell. This applies to everything from plants that aren’t fully healthy to varieties that aren’t clicking with customers and everything in-between.

“I have not heard anybody say that they aren’t doing well,” he says.

Up-tempo production

When production is moving that fast, Palmateer says it is important to still have a strategy in mind to prevent pest and disease issues. Just because plants are leaving the greenhouse sooner doesn’t mean they are immune to pest pressure or disease. It also might mean they rely more on insecticides or miticides instead of a plant growth regulator (PGR).

“Some growers will want to take something that is highly systemic and has a higher residual and use that,” he says. “But they might only have to treat one time before selling. There are other cases where you are battling flare-ups you have to deal with in a short period of time. Because of the diversity of ornamentals, though, it really depends on the growers and the type of crop.”

Palmateer also cites mealybugs as an illustrative example of an issue impacted by shorter production cycles. Before the pandemic, he notes that it was a “hot problem” and hard for growers to get a handle on it. And while it is still an issue in an area like South Florida, plants are spending less time in the greenhouse and not running up against mealybugs at a noticeable level, if at all. He says the same has been true for thrips over the last year-plus.

“This shorter production cycle really just requires less input,” he says. “But it really comes down to how long crops are being held before being shipped out.”

Palmateer’s role at SePRO

In his new position at SePRO, Palmateer says one of his major roles will be supporting the company’s sales team by working with growers and the overall product lineup. The hope, he says, is to limit confusion growers have and to help them find the right product for their operation.

Another major responsibility is working with both research and regulatory to continue to expand the product portfolio. As new products are developed from research, he’ll be working with SePRO products in development in trials at both universities and in the private sector. On the regulatory side, he’ll help write SePRO labels to meet government regulations, but also make the labels easy to read for growers.

Palmateer also notes that, in working for a company like SePRO, he likes collaborating with others and operating in a team environment. During his time as a professor, he says a lot of his work with self-started and done on his own. Here, he can be part of something bigger.

“My role is going to touch every part of the business,” he says. “It’s got some research and development work; it’s got developing labels and working with marketing and sales. I’m really excited to get going. Everyone on this team is driven for success and everyone benefits from everyone’s success.”

© vladteodor | Adobestock

When pests are discovered in the greenhouse, they were likely reported by a scout.

The first thing you need to consider when you think about scouting is to come to grips with who’s actually going to do it. Some growers have full-time scouts, some have part-time scouts, but for most operations scouting is part of someone’s responsibilities — not an entire job description on its own.

“Don’t think you have to have a paid, titled, credentialed scout do the scouting,” says Timothy Malinich, horticulture educator in agriculture and natural resources with the Ohio State University Extension’s Erie County office. “Scouts can just be people that have been empowered to go out there and look for things, see those things and report them back to the people that will need to treat them.”

Scouts could be site managers. If a greenhouse has multiple locations, it could be the manager of each site. It could be all employees at the greenhouse. It could be the employees on the potting line or employees managing inventory in the shipping yard. In many cases, it’s the applicator.

Greenhouse managers can empower everyone in their organization to scout, but you need to make it part of the job description. You must give the people you are empowering to scout time to do the work and be flexible about when they do it.

You should also provide some basic training on what scouting entails. Though even without formal training, if an employee sees something that doesn’t look normal, they can report it and you can use it as a point of data.

Thomas DeHaas, an agriculture and natural resources educator at Ohio State University Extension’s Lake County office, says every employee is likely to have one of the most powerful scouting tools right in their pocket: the smartphone.

“They can take a picture and text somebody,” DeHaas says. “If you have a central person who gets that info, you’ve already improved your scouting program 100%.”

DeHaas suggests growers think outside the box to incentivize employees to scout in addition to their responsibilities. For instance, you could offer $1 for each picture sent that turns out to be a problem.

“If it’s worth doing, it’s worth paying for,” DeHaas says. “Giving people time to do it is going to cost you money, but it saves money, too.”

There are several financial and plant health benefits to a robust scouting program. Because your trained scout or scouts can find pests before they become a problem, you’re able to reduce pesticide inputs, preserve your beneficial insect supply, and reduce the chance of pesticide resistance.

Scouting tools

The tools of the trade can vary widely in price and effectiveness. If you ask a large amount of your employees to scout, you won’t outfit all of them with Canon Rebels and pricey magnifying lenses. But everyone should at least have the basics listed below. Here are some options:

Basic tools

Smartphone, loupe, pen and waterproof paper, pruners, marking tape, flags, acetone with cotton balls, hand sanitizer, map of greenhouse (to mark where pest was found), yellow sticky cards.

Midrange tools

Digital magnifier, EC/pH meter, clip-on lenses for smartphone (ex. CamKix), Hand-lens with 10x power and/or optivisor (headset with magnifying glass), Resource information such as pesticide labels, pictures and life cycles of key pests.

High-end tools

Field microscope, “real” camera (D-SLR style), laptop for image manipulation, clip-on lenses like olloclip.

Source: Reporting by Matt McClellan and information from the University of Massachusetts Amherst

Emma: Weed control is a constant battle for greenhouse and nursery growers. Can you tell us about some of the key weed species we run into in indoor production and why they are problematic?

Chris: Despite our best efforts, weeds will usually find their way into a greenhouse by some way or another. The greenhouse is a perfect environment for many different weed species but certain species like bittercress (Cardamine spp.), woodsorrel (Oxalis spp.), and liverwort (Marchantia polymorpha) can really thrive and spread exponentially inside a house. Bittercress and woodsorrel grow fast, produce many seeds, and the seed pods explode when they are ripe, spreading the weeds throughout the house. Liverwort is problematic mostly because it loves two things: moisture and nitrogen. Propagation mist beds are perfect for liverwort. Liverwort spreads both vegetatively and via spores, and can spread throughout a house extremely quickly. It is also almost impossible to hand weed. Once any of these species gets into the actual crop, there isn’t much that can be done other than manual removal.

Emma: So frequent scouting, along with strict sanitation practices, is critical to keeping weed populations in check. Despite our best efforts, sometimes weeds can get the upper hand and unfortunately, there are relatively few herbicides that are labeled for use indoors due to volatility concerns.

Chris: Certain herbicides that have high vapor pressure can turn into a vapor after they have been sprayed (or applied as a granular) and these vapors can injure plants. This is problematic for fully enclosed greenhouses because the vapors get trapped inside. Whereas something like a shade house (only covered in shade cloth) doesn’t pose the same concern because vapors can dissipate more easily. Plants that are treated with preemergence herbicides and moved inside a house soon thereafter are also susceptible to volatility injury. Ensure that the herbicide that is being used inside the house is labeled for that use. Also, make sure to time your preemergence application so that it is made at least 2 to 3 weeks (depending on label) before you have to move the plants inside.

Emma: Herbicide options for enclosed structures are fairly limited but there are great options available. Post-emergent herbicides like glufosinate (Finale® XL T&O herbicide) can be used indoors while plants are present. This allows for cleaning up weeds under benches, along walkways and around equipment.

Chris: Other post-emergent herbicides include diquat (Reward) and Scythe or Axxe (pelargonic acid/ammonium nonanoate). These are all contact or minimally translocated herbicides so thorough coverage is needed for control. Be sure to turn off air circulation fans during the application. For grassy weeds, clethodim (Envoy Plus) and fluazifop-butyl (Fusilade II) could be used. Indaziflam (Marengo) is a broad-spectrum preemergence herbicide that could be applied either alone or in a tank-mix with a postemergence herbicide on the floors or under benches to prevent weeds from coming. When the house is empty (and will be for 24+ hours), you have a few more options such as glyphosate (RoundUp Pro and others) for postemergence control and flumioxazin (SureGuard) for preemergence control.

Emma: When we start talking about herbicides for outside the greenhouse or even outdoor production areas, we definitely have more options. What should growers consider when selecting herbicides for outside and around the greenhouse?

Chris: Poor weed control outside of the greenhouse will inevitably lead to poor weed control inside the house. Frequent mowing, weed fabric, gravel, basically just general sanitation, are good practices to keep weed pressure low. Most of the postemergence herbicides labeled for use in and around ornamentals in nurseries can be used around a greenhouse, but the main thing you want to avoid is the use of volatile herbicides that could drift into the house through doors, vents, etc. A major herbicide group to avoid is the auxin type herbicides like 2,4-D and dicamba. These are mostly used in turf and can be prone to volatilization. Soil sterilant herbicides like dichlobenil (Casoron) should also be avoided when treating in close proximity to production areas. It is always a good idea to shut vents and windows when treating large areas around the house.

Emma: In addition to weeds being an unsightly greenhouse pest all their own, weeds can also serve as harbors for insects and other pests and can act as additional hosts for some plant pathogens like impatiens necrotic spot virus (INSV) and tomato spotted wilt virus (TSWV).

Jen: Greenhouse weeds also provide a significant reservoir for insect and mite populations to build and thrive. They can be missed with spray applications, and even fogs and beneficials may not reach spider mite nurseries under the bench, or thrips, whitefly and aphid populations living in weedy corners or behind greenhouse structures. Don’t forget that algae is also a greenhouse weed, and it feeds and harbors shore flies and fungus gnats, allowing them to lay more eggs and be present in the range all year long. These flies and others associated with wet conditions, like drain flies, are not only plant pests and nuisance pests as they dart around workers’ faces, they can also move pathogens between plants and crops.