Greenhouse Media for Growing Hemp Cuttings

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Greenhouse media for growing hemp cuttings

Keith Edmisten, Carl Crozier, and Anthony LeBude 
North Carolina Cooperative Extension
Kristin Hicks, North Carolina Department of Agriculture & Consumer Services

Posted on 6/26/2019

North Carolina has a large container plant industry, and hemp producers should take advantage of this local expertise. Dr. Anthony LeBude from the Horticultural Science Department at NC State University offers the following advice:

Plants without roots will not benefit from fertilizer in the substrate until they have roots, but as soon as they have roots, the rooting environment is the worst place for photosynthesis because of the super high humidity, so it’s best to transplant them. It makes sense to fertilizer with a controlled release fertilizer (CRF) that won’t release until the cuttings root in 6-8 weeks (for woodies), but it never really works out that way in real life. It’s best just to fertigate when rooted and transplant as quickly as possible to a more suitable growing environment.

Several growers have found traditional tobacco type soil media to stay to wet for hemp cuttings, often resulting in severe disease outbreaks. These growers have had more success with dryer media, such as coconut coir.

Another problem we have seen with injury to cuttings in greenhouse seems to be related to salt injury due to high electrical conductivity (EC). Dr. Kristin Hicks from NCDA&CS has found some media that has been tested had high EC due to high Ca and S levels or high Na Cl levels. She suggests that growers test their media for nutrients and other parameters that may affect plant health. Dr. Hicks found the media used in the plant below to have excessive Ca and S levels.

Image of salt-injured hempDr. Anthony LeBude explains: There is no reason for trying to vegetatively propagate unrooted stem cuttings in substrates with high electrical conductivity (EC). The plant must try and pull water away from all the salts present in the substrate without roots, and to so this it needs to dry itself out, so it can be burned easily through desiccation, or simply have the stems burned by too much fertilizer. When there are problems, it’s possible that the producer used a “secret recipe” for the propagation substrate that includes some sort of manure, gypsum, fertilizer, or other supplement that is high (hot) in terms of EC. Flushing the substrate over several days is the best practice here to try and lower the EC. Producers can also simply make a new substrate and transfer the cuttings, it won’t hurt them as long as the new substrate is moist. They could even take the time to re-dip the cuttings in rooting hormone and see if the portion of the cutting that was inserted into the substrate was burned off to the soil line, which happens a lot with hot substrates.

Coconut coir can also be high in salts because it is high in sodium to begin with, but producers of coir try to remove the high sodium by exchanging with calcium and magnesium. These are also salts, but “harm” tissues less than sodium does on average. For plants with roots, the high calcium and magnesium might not be a problem, but for unrooted cuttings it can burn the stems inserted into the media. Make sure the coir supplier offers a specification sheet on what the electrical conductivity of the coir is after they processed it. Some will mention they flush it with water or exchange it with calcium and magnesium. In all cases, unfortunately, the plant grower is the final quality control manager and is responsible for checking the EC themselves using this simple procedure. If growers want to also test the porosity of their substrate, they can use this simple tool to give a “ballpark” figure to compare possible substrates in terms of how much airspace they have after watering. Finally, growers are encouraged to periodically test their media for nutrient content and parameters such as sodium, chloride, pH and EC. Samples can be submitted to the NCDA&CS Soilless Media Lab for a fast, low-cost analysis.

Reviewed and updated by Jeanine Davis, Department of Horticultural Science.