Hunter Cobb, Carolina Greenhouses’ outside operations manager, in the business’s hemp crop.
Photos courtesy of Carolina Greenhouses

Growing hemp in a greenhouse can offer numerous benefits, such as light and climate control, year-round grows, and security, just to name a few. Below are four tips that can help reduce grow issues once seeds or clones have been planted.

1. Ventilate, ventilate and ventilate.

Whether you are producing transplants or a finished crop in your greenhouse, you must have ample ventilation. Many areas in the U.S. deal with high humidity. You need enough mechanical ventilation to remove moisture in the greenhouse and facilitate CO2 exchange. If you do not have enough air movement across the plant canopy, you may run into issues such as powdery mildew and botrytis. For houses less than 150 feet long, fans located in one end wall with the appropriate square footage of louvres (wall shutters) at the opposite end seem to work. However, when houses are 150 feet or longer, we have found it better to place large fans on a sidewall at the midway point and pull outside air from each end-wall shutter. Our rule of thumb is that every plant in the greenhouse should be “dancing” (moving or swaying) constantly from air circulation.

2. Maintain proper pH and EC.

Choosing a quality growing medium is crucial since it will impact the plant throughout its entire growth cycle. Carolina Greenhouses uses soilless mixes with a pH between 6.3-6.5, good draining ability, high air porosity and effective water retention. Many good mixes have an assortment of bacteria and fungi that support plant growth. For our plant production, we use a mix that has a 65:35 peat-to-perlite ratio; this has given us our best yields to date.

You should always know what the pH of your media is during the entire growth cycle. Use proper PourThru techniques to check pH, and electrical conductivity (EC) readings. Ideal EC readings depend on what growth stage the plant is in, but proper EC and PH are both critical for a happy plant. (Editor’s note: Read more information about the PourThru method). We always make sure our solution tanks, where we mix nutrients with source water, have the proper pH and EC readings before we pump the water through the irrigation system. Every time we water or feed, we do this—not occasionally, but always! Hemp does not like to be overwatered nor overfertilized.

3. Do not skimp on genetics.

Start with good genetics. Whether you use seed or clones, genetics combined with proper nutrition have a huge impact on the quality of hemp grown in your greenhouse. Purchase clones from a reputable company that is willing to be open about its cloning process and mother plants. The same goes for seed. Make sure the company has proven genetics and is able to provide data on testing and the history of the variety.

4. Provide adequate plant spacing.

One of the biggest mistakes we have seen other hemp growers make is packing too many plants in a greenhouse. Hemp plants do not like to be overcrowded. This can cause various issues, such as pest infestations and yellowing underneath the canopy. Too many plants can also hurt yields. It’s better to err on the lower side than it is to overfill the greenhouse with plants that you cannot get to and properly care for.

[For tips on transplanting hemp in the field, read part I of this two-part series.]

Lisa Cobb is president of Carolina Greenhouses, a company that has grown agricultural crops such as tobacco for 30 years and incorporated hemp production when the 2018 Farm Bill passed.