Growing media use ranks second among nursery production in Miami-Dade County, estimated at around 10% of the overall production costs. Growers are interested in finding ways to reduce their production costs, including the use of growing media, particularly when the supply of peat becomes unreliable. The recent development of the substrate stratification technique is one approach aimed at addressing this challenge.

The concept ‘substrate stratification’, first published in 2021 by Dr. Fields et al., was designed to reduce the use of more expensive growing media (peat-based) by placing cheaper and coarser materials at the bottom of a container. Depending on the crops, you can apply a cheaper material up to 50% of a container without negatively affecting the plant performance (Pictures on the right and below with Coleus grown in a stratified system with fine bark).

In addition to the reduced use of peat, stratification can reduce the water content in the bottom of a container because the coarser material has less water-holding capacity than peat, which may decrease the incidence of soil-borne diseases. Moreover, the stratification system can make it possible to reduce fertilizer use, for example, you can apply only 80% of the total amount of CRF that is designated for the whole container into the top layer without affecting the plant growth.

The roots tend to concentrate in the top layer at the early stage for a stratified system, but they will eventually grow to the bottom of the container with even stronger roots because of more air space at the bottom than in the non-stratified system. When looking at the canopy size, however, there was no apparent difference between stratified and non-stratified systems in recent trials conducted in Miami-Dade with crops including Coleus, Hibiscus, Ixora, Mandevilla, and Vinca (See the Mandevilla plants below, and Coleus has bigger canopy grown in 50% stratification).

On the other hand, the particle size of the coarse material used for stratification, such as aged pine barks, can affect the growth of the root ball into the coarse material at the bottom. The bigger the particle size, the slower the growing speed of a root ball toward the bottom of a container (Picture on the left: Coleus grown in fine pine bark for 2 months with the root reached to the bottom; Picture below: Mandevilla grown in coarse pine barks for 3 months and root ball not fully formed yet in stratified substrate). Thus, the fine pine barks (through 4-6 mm screen) may be better suited for crops with a short growing period after transplant, such as Coleus and Vinca, while coarse pine barks (through 9 mm screen) or larger (pine barks for landscape use) could be used for crops with a longer growing period after transplant (Hibiscus or Ixora). Small trials may be conducted for a specific crop to optimize the particle size or the ratio between two particle sizes.

Here are things to consider if you want to test this technique with your specific crop:

1. Material to consider:   Compost pine bark is the most studied material to date.
2. Particle size selection:  Smaller size (fine) for short growing crops, larger size (coarse or larger) for longer growing crops.
3. Ratio for stratification: up to 50% of a container (May need to conduct a trial for a specific crop).

Credits: Some of the pictures used in this post are adopted from presentations of Drs. Fields and Dr. Jackson.

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