The Science of Substrates
One of my favorite topics in horticultural production is soilless substrate (media) hydrology and physics. The majority of the research I have done has involved water conservation in container agriculture. While there are many opportunities for success in this field, I have focused primarily on how water is retained and distributed within the soilless substrate. I followed that with incorporating various irrigation systems to understand how soilless substrates and irrigation scheduling can work together to save water, fertilizer and, most importantly, money.
When growers first started using containers to produce plants, they noticed that the water did not drain well in mineral soil that was used to fill the containers. This phenomenon is known as the “container effect” and is in part a result of the small particles of the soil creating a pore structure that is unable to overcome the effect of the height of the container. The relatively short height of the container (compared the depth of the soil profile) does not provide sufficient gravitational
force to overcome the suction of the pore structure, inhibiting proper drainage. To counteract the “container effect,” soilless substrates were initially designed to allow for ample drainage regardless of the volume of water that was applied. This meant that no matter the conditions, an adequate volume of air would remain within the substrate. Thus, much effort has been put forth to help growers overcome the container effect while providing a sustainable water and air balance, and the results are countless variations of soilless substrates used throughout the industry. Today, soilless substrates are often composed of regionally available organic materials. In the southeastern United States, aged pine bark is the primary substrate component because of availability, relative low cost and, most importantly, the desirable physical properties. In a container, pine bark provides a pore structure that does not inhibit drainage. This means that the plant roots will always have sufficient airspace for healthy growth and vigor while allowing for adequate water during and between irrigation events under normal production circumstances.
Historically, we have categorized soilless substrates through what we consider static physical properties, which are properties that are stable during and between irrigation events. Some commonly measured static physical properties are the maximum water-holding capacity (container capacity) and minimum air-filled porosity (air space). We measure maximum water and minimum air space as a balance to assess the total pore space (total porosity) of a substrate. As I mentioned previously, substrates were initially designed with these static physical properties in mind to be very forgiving to growers, who in the past were often not faced with the ecological and economic issues associated with high water usage we face today. In fact, these static physical properties are almost exclusively used to characterize and develop soilless substrates over the years. However, substrate physicists like myself are now focusing more on incorporating dynamic hydraulic properties into substrate characterization. As a plant receives irrigation, the water content changes within the substrate over time. Similarly, after the irrigation event ends, the water slowly drains away. In addition, we know that the water within the substrate does not just move up and down throughout the profile or in and out of the container. Instead, there is lateral movement and redistribution constantly occurring within the substrate. There is also channeling and both water and nutrient gradients within the profile of the container. Therefore, with increasing ecological and economic challenges facing our industry in the 21st century and beyond, it is imperative that we investigate and incorporate dynamic properties into our respective programs to increase efficiency throughout the entire growing process, as opposed to using properties that only represent real world scenarios when the substrate is at maximum water-holding capacity, which rarely is achieved under normal production scenarios.
To understand the dynamic hydraulic properties of the substrate, we must understand three major concepts: water content, water potential and hydraulic conductivity. Water content is straightforward. This is the percent of the volume of the container occupied by water at any moment. The water content of the substrate is in constant flux with additions (irrigation and precipitation) and subtractions (drainage, evaporation and transpiration) constantly occurring. Water potential is a bit more complex and is comprised of various aspects, including fertility, gravity and water concentration, among others. Water moves from high concentration to low concentration, and water potential is essentially the relative tendency of water to move from one area to another. Finally, hydraulic conductivity is essentially how easily water can move through the substrate. These three properties are inherently entwined, and any changes made to one of the properties will directly result in changes in the other two properties. Therefore, as the water content of the substrate is constantly increasing and decreasing, the hydrology of the substrate is constantly fluctuating. So, understanding the container substrate system throughout the production process is key to designing not only substrates, but irrigation and fertility systems that will work for the specific crop.
Using dynamic properties to develop soilless substrates, growers have been able to produce more sustainable container substrates, which can have a plethora of associated benefits. We can design substrates that let plants access higher proportions of the water held (available water), which will allow fewer or lower-volume irrigations. We can improve the “delivery” of water to the roots and allow plant access to water from previously inaccessible spaces in the container. By ensuring that higher proportions of water are available to plant, we can reduce irrigation requirements. Plants can be grown with extremely low leaching fractions, which is the proportion of water that gets into a container and drains away. Reducing leaching not only saves water but also reduces costs associated with fertilizer applications. Low-water irrigation systems, when paired with the properly optimized substrates, have been shown to not only reduce water and fertility requirements, but can also encourage increased growth rates, resulting in reaching marketability sooner.
These are just a few of the potential outcomes of incorporating dynamic hydraulic measures into substrate development. Understanding the relationship between the substrate, water and fertilizer in a container and knowing the movement within will allow
for continued beneficial improvements both economically and ecologically for our production horticulture industry. Moreover, these relationships and dynamic properties can be applied to amended and raised landscape beds to provide similar benefits and improve plant health.
Dr. Jeb S. Fields is an assistant professor and extension specialist with the LSU AgCenter located at the Hammond Research Station. Dr. Fields runs the Ornamental Trial Gardens at the Hammond Research station, and his research and extension activities generally focus on resource efficiency and abiotic stress in container crop production.
Where are our Olive Trees Now?: Winter 2018 Olive Update
As 2017 came to a close, the Hammond Research Station wrapped up three years of evaluating trees producing edible olives for suitability under Louisiana’s growing conditions. Encouraged by the success of olive orchards in south central Texas, 15 varietals of Olea europaea were observed at the station for establishment success and resilience against the harsh weather our state can throw at plants. Olives are primarily used for orchard production, but some varietals are also used as ornamental specimens in the landscape.
The Hammond olive trees endured two historic floods, high humidity and wild swings in temperatures. In December 2017, a 6-inch snowfall blanketed much of southeast Louisiana, causing the olive trees to resemble bright white weeping willows. As soon as the snow melted, the flexible branches of the trees returned upright with no signs of damage. By the time we had reached the end of the study, six varietals stood out as the toughest selections, and another four performed well but suffered greater cold damage during the 2016-2017 winter.
Little did we know that early 2018 would provide us with an unfortunate additional test for cold hardiness in Louisiana! The week of January 15, 2018, brought record low temperatures along with a healthy serving of ice and sleet for some parts of the state. Temperatures at the Hammond Research Station dropped to 14 degrees Fahrenheit and stayed at or below freezing for 24 hours the first full day of the arctic blast. Although we luckily missed most of the ice accumulation, these temperatures proved too cold for most of the olive varietals.
While we know that olives do not like temperatures below freezing, some sources note survivability down into the mid-to-low twenties. Indeed, Texas and southeast Georgia have healthy olive orchards that have survived occasional cold blasts.
Factors determining resilience include the age and size of the tree. Specific varieties are also more resistant to the cold. Also, in general, older trees with larger trunks tend to receive less damage during cold snaps. Although our trees had been in the ground for three years, many of the varietals died completely or were killed to the roots. The canopies exhibited a high degree of bark split along with heavy defoliation. We did not provide any cold protection for the trunks or canopies because this was an establishment trial.
Some varietals, however, suffered little to no bark split or defoliation. We were amazed by their performance and have revised our list of recommended olive varieties to now include only four: Anglandau, Arbequina, Bouteillan and Picual. All the survivors are French or Spanish varietals commonly used in orchards, and all but Arbequina have silver-gray foliage, making them useful as ornamental specimens in the landscape.
While unfortunate for our olive trees here at the station, these kinds of killing temperatures provide valuable information that help people understand the risk level associated with investing in any new specialty crop. This past January reinforced our initial recommendation that anyone wishing to start an olive orchard should find such a location south of the Interstate 10/12 corridor in Louisiana. Likewise, the trees need a lucky run of at least three years of milder winters to grow to a larger size to withstand such cold temperatures. In south Louisiana, where soils tend to be heavier, proper elevation of the planting rows will be key to providing the trees with good drainage. In general, though, weather patterns tend to favor production farther to our west in Texas, so any large investment in olive production in Louisiana needs to be cautiously evaluated.
Using olive trees in limited numbers in the landscape comes with less financial risk, as Louisiana enjoys many ornamental plants that have only marginal survivability here during record cold weather events. The aesthetic qualities of olive trees are unique and often not represented by any other plant material grown in Louisiana.
The bottom line? Proceed with caution, plant only in south Louisiana and try to choose one of the four varietals that demonstrated the greatest survivability in our trial!
Sedge Control in Landscape Beds
Sedges are extremely common weeds found throughout landscapes in Louisiana. The most common sedge species infesting landscape beds are purple nutsedge ( Cyperus rotundus), yellow nutsedge ( Cyperus esculentus) and kyllinga ( Kyllinga spp.). Sedges can be found in nearly all soil types and growing environments but thrive on conditions found in flowerbeds. The plants’ upright growth habit and dark-green color (purple nutsedge) or pale-green color (yellow nutsedge and kyllinga) make the weeds a prominent distraction in the aesthetics of a high-quality landscape. Although grasslike in appearance and often referred to as “nutgrass,” sedges are not grasses at all. Grasses are members of the plant family Poaceae. In contrast, sedges are members of a totally different plant family, Cyperaceae. Sedges can be identified by their triangular stems, while grass stems are flat or oval. Distinguishing between grasses and sedges is very important because successfully removing sedges from landscape beds requires selective sedge-killing herbicides. Most other herbicides either have no effect or only slightly injure the weeds.
Yellow nutsedge is often confused with purple nutsedge. As their names imply, yellow nutsedge produces yellow flowers and purple nutsedge produces purple flowers. Flower color makes identifying nutsedges simple. But how do you identify nutsedges when no flowers are available?
When there are no flowers, leaf tip is the most direct diagnostic characteristic to differentiate the two at very early stage. Yellow nutsedge leaf tips have a long and tapered point (spear shaped), whereas purple nutsedge leaf tips are bluntly pointed (dagger shaped). Both species produce rhizomes and tubers, but purple nutsedge tubers are connected together with chains of rhizomes. Yellow nutsedge produces tubers at the end of rhizomes. If you still can’t tell whether you have yellow or purple nutsedge, yellow nutsedge tubers taste like almonds. Purple nutsedge tastes bitter. So far, no one has taken me up on the “tasting tubers” method of sedge identification. Perennial kyllinga species only produce rhizomes. Sedges can differ in their susceptibility to herbicides, so distinguishing between species is critical for management decisions.
Sedge Control Options
· Pre-emergence herbicides, such as metolachlor (Pennant), dimethenamid (Tower) and dimethenamid + pendimethalin (Freehand), only have activity on annual sedges and yellow nutsedge. See product labels for use in bedding plant areas.
· Halosulfuron (Sedgehammer) provides good post-emergence control of purple and yellow nutsedge and suppression of kyllinga. Spray halosulfuron on sedges around established woody ornamental species in landscaped areas.
· Sulfosulfuron (Certainty) is a good post-emergence herbicide on most sedge species found in flowerbeds, including the kyllinga species. Sulfosulfuron can be applied around woody ornamentals as well as directly over the top of several perennial ground covers, including mondograss, Liriope muscari Big Blue, Liriope muscari Variegata, jasmine and others.
· Imazaquin (formerly Image, but now called Scepter) provides good control of several sedges and may be applied over the top of liriope, mondograss, jasmine and several woody shrubs (see product label). Do not apply around or over the top of bedding plants.
When it comes to controlling nutsedge in flowerbeds, always keep in mind that nutsedge species rank among the worst weeds in the world. Purple nutsedge is the No. 1 weed in the world, and yellow nutsedge ranks 16 th ( Yes, they actually rank weeds!). So, rally the troops! You have a very formidable opponent. Let’s go kill some sedges!
Yellow nutsedge is often found irrigated landscape beds
Purple nutsedge produces chains of tubers making hand removal difficult
Yellow nutsedge leaf on the left versus purple nutsedge leaf on the right