Which growing substrate for paper-wrapped pots?

Many different growing substrates work well in paper-wrapped propagation pots such as EllepotsTM. Consider the following pointers:


Components typically include peat, bark, coconut fiber (coir), perlite or vermiculite. Most paper-wrapped pot substrates are peat-based (water and nutrient holding) with around 70% peat/30% perlite by volume being a common blend. Coir may improve wettability. Bark and perlite can increase aeration. Vermiculite may increase nutrient holding.

Particle sizes:

Because paper-wrapped pots are typically more compressed than loose-filled trays, growers usually select a coarse “finished plant mix” designed for propagating cuttings or finishing plants up to 8-in (20-cm) diameter in a greenhouse. Products labeled as a “seedling plug mix” are likely to be too fine. If particle size changes, then the amount of water held in the pot will also change. The substrate should not be so fine that the substrate holds too much water and not enough air, or the plant roots will rot. The substrate should not be so coarse that sticks or long fibers clog up the machine or particles fall out the bottom. Most particles should be 0.5 to 5 mm. Avoid sticks (larger) or dust (smaller) particles. Particle size can change from one batch of delivered mix to another, so visually evaluate particle size or do a sieve test. Check our Testing particle sizes protocol in (BackPocketGrower.com, “BPG”) on how to screen for and interpret particle size.

Hand testing particle sizes

Moisture level: 

Moisture level affects how the substrate moves through an EllepotTM machine. Too wet, and the machine will clog up. If the substrate is too dry, mix will fall out of the bottom of the pot. Dry pots are also hard to rewet by the grower. Very wet substrate will be more compacted during pot production, resulting in less air inside the pot. Compact substrate also means more substrate is used per tray, increasing cost. When wet pots are stored, fungus is likely to grow resulting in grower complaints. Wet trays weigh more, increasing shipping cost. The moisture content for Ellepot substrates needs to be around 60-70%. The Look and feel method in BPG provides procedures for testing substrate moisture content.

Growing substrate too dry

Tray growing fungus because too wet

Hand demonstrating look and feel method

Air and water porosity: 

Substrate mix needs to provide the right balance of air and water after irrigation. Test air and water porosity following our Porosity testing (air and water) protocol in BPG.

Growing substrate


It is important that the substrate absorbs water easily. Otherwise it will be hard to add water to substrate in the hopper, and pots will dry unevenly in the greenhouse. The Substrate wettability tests in BPG have the detailed procedures of how to test this feature.

Nutrient level:

  • With each new batch of mix, send off a sample to a soil testing laboratory
  • Test pH (usual range of 5.6 to 6.4) and electrical conductivity (EC, which has a usual range of 1.0 to 2.5 mS/cm) on site as described in our Plug squeeze protocol in BPG. If pH or EC are out of range, plants may show nutrient deficiency or toxicity. 

Hand using the right pH testing method

For more information: Contact authors Jinsheng Huang and Paul Fisher of University of Florida IFAS Extension, and Dr. Bill Argo of Blackmore Co. Thanks to our Floriculture Research Alliance at University of Florida sponsors including A.M.A. Plastics, Blackmore Co., Everris, Fine Americas, Greencare Fertilizers, Klasmann-Deilmann, Pindstrup, Premier Tech Horticulture, Quality Analytical Laboratories, Sun Gro Horticulture, and leading young plant growers. The University of Florida does not endorse any product, and our research focuses on quality testing on these and competing products to assist grower success. June 30 2015.