To stratify or not to stratify?

By now you have probably heard of this new concept in stratifying substrates or creating layers of different substrates in the container. You might ask — Why would I want to separate my container’s vertical profile into two halves? Our research has shown the following benefits to your crop and bottom line:

• extend or reduce use of premium substrate
• quicker crop establishment
• rid yourself of soggy container botoms that are hard to dry out
• greater root growth
• reduce crop stress between irrigation events
• use less controlled release fertilizer
• extend water supply by irrigating for less time

Five years ago, we were simultaneously working on projects that looked at crop response in fine and coarse textured substrates, bioretention systems for landscapes, isolating fertilizer in the top half of the container, and defining unsaturated hydraulic conductivity in bark-based substrates with different particle size and fiber additions (coir and peat). Each project resulted in an improved understanding of a component or process in the container, helping growers incrementally along the way. Then, one day, a light bulb went off when we had the crazy notion of combining all of these concepts into a single holistic system. We had the idea to intentionally stratify each container to incorporate all the benefits we found in the aforementioned projects. We found that these layered, or stratified substrates, produce a starting point to slow water movement, ensure water storage where it was needed, increase mineral nutrient availability, decrease water and agrichemical leaching and increase gas exchange in the lower part of the container to support healthy root establishment.

Why not advantageously use each individual pot to get the most “bang for your buck?”

We hit the ground running to implement and understand stratified substrates with Kris Criscione, doctoral candidate at LSU AgCenter, doing some of the heavy lifting along the way.

In the field

We started with some mini evaluations, and after our preliminary trials we went big. We invited James Altland and Chris Marble to install large trials at Bailey Nurseries and Woodburn Nursery in Oregon with the help of Phillips Soil and Harrell’s. Our findings:

• We showed across taxa, size, and nursery (irrigation) that the most economically important taxa including rose, arborvitae, and hydrangea performed similarly or better when a fine substrate was placed atop a coarse substrate with CRF incorporated in only the top half.
• We also found that approximately two inches of coarse substrate atop conventional suppressed establishment of liverwort.

We then decided to move from the farm to the research plot to pinpoint specific benefits and limitations of the stratified system. In our first replicated research trial funded by the Horticulture Research Institute, we compared stratified to conventional substrates when producing shrub roses with 20% reduction in both fertilizer and irrigation. Rose in stratified substrates established quicker and grew of equal or greater size with less water or fertilizer while reducing time to market by 25%. We followed that up with a few other production projects, with one of the key takeaways being greater root mass when grown in stratified versus conventional substrate. We continue to see these improvements in research evaluations today.

In the lab

We found that the vertical moisture gradient in stratified substrates becomes more uniform from bottom to the top of the pot, and most importantly, the top portion of the substrate has less daily moisture flux than in conventional substrates. The more stable moisture keeps the young plant hydrated while still providing drainage below the plug or liner that ensures oxygen as the roots grow downwards. Additionally, water infiltrates the finer or fiber-amended top substrate more slowly, aiding in horizonal wetting. In all studies we saw equal or increased root growth across taxa.

Extending your premium substrate

The greatest financial impact was the use of stratified substrates to extend peat or peat-based substrates in greenhouse and some nurseries. The best part of our jobs as researchers is working with growers to solve problems, and this is a clear example of that collaboration paying off. With our fixation on reducing water, fertilizer and production time, we didn’t see what might be one of the biggest improvements until Tony Carter of Langridge Plant Sales in Belle Chasse, Louisiana asked us, “why can’t you use this new technique to lower soil costs?” Of course, we instantly followed up in the lab and found that filling the pot with approximately 50% bark before filling with peat-lite resulted in healthy plants with greater root growth across the vertical profile after roots first filled the top portion of the container. Growers can now use commercial peat-lite mixes atop conventional bark to reduce costs, ensure adequate peat supply for the duration of the season, and increase water buffering in their soilless substrates to reduce plant stress.

The most asked question: How do I implement in potting line?

Nursery and greenhouse operations have been able to easily stratify when hand filling pots by simply having the two substrates on hand, others have run pots through filling lines at twice the normal speed to fill the lower half of the container before filling the top of the container, while others are having equipment built to fill container using two-hopper or two-conveyor systems. If there is a will, there is a way. We suggest you try out stratified substrates to see it is easy for you and work for all your crops.

Hurdles for trialing and adopting stratified substrates

The primary hurdle to trialing and adopting stratified substrates is the logistics and willingness to adjust your cultural practices to see the value-added benefits of crop uniformity, water saving, fertilizer savings and reduced time to market. Then after successful trialing, you must figure out how to adapt your potting line. There is virtually no risk to crop health and growth rate. To date, we have killed no plants in our trials – and that is saying something for us. In fact, we have found plants of equal or greater size in almost all trials. We have also found that stratified systems are very robust, and do not require very exact specifications to provide benefits. Follow the guidelines provided above, and even with generous variation in your approach, you will still see the benefits of stratifying substrates.

As this research continues to develop, we continue to build upon our past work and find new benefits, methods, and opportunities for using stratified substrates. But we know there are more big ideas out there to make the substrate and container system better. If you are interested in stratification, give us a shout. We would love to chat with you or your production crew about substrates, irrigation, fertilizer or water management. We want to support your implementation or on farm trials as best we can, as that is the best way ensure our research, including stratified substrates, becomes an industry staple.

Drs. James “Jim” Owen Jr. (jim.owen@usda.gov) and Jeb Fields (jsfields@ agcenter.lsu.edu) have worked together for over a decade to understand and improve soilless substrates utilized in greenhouse and nursery production. Jim and Jeb are the leaders in stratified substrates and are always happy to talk with growers. Reach out to either of them for collaborative research or knowledge on how to test or implement stratified substrates.