Photos courtesy of Marguerite Bolt
Square and octagonal tray cells often have grooves, which encourage downward root growth and provide a good anchor for the plant.

Whether hemp will stay indoors the entirety of its lifecycle or be transplanted into the field (which many cannabinoid hemp growers choose over direct-sowing for higher success rates), selecting the right trays and pots for growth is important. However, what works for one operation may not quite fit a different operation. Cultivators have several factors to consider when selecting containers that fit their budgets and production needs.

Getting the Right Start

Growers have many options when it comes to containers for starting seeds or cuttings. Plastic propagation trays, or flats, are a common option because they are relatively inexpensive and easy to find.

The goal of a tray is to provide a place for roots to develop. Those plants should then be moved to a larger container or the field before roots bind up. The most common propagation trays for hemp production consist of multiple cells, providing a spot for individual seeds or cuttings. The depth of the tray, the shape of the cell, and the width of the cell are common variables growers need to consider.

Trays are often sold in 50-cell or 72-cell options with variations in the cell depth and shape. The total width and length of 50- versus 72-cell trays will typically be the same; but with more cells, a 72-cell tray will have much smaller compartments for the plants. This may work for plants that will remain indoors, but for outdoor production, having a 50-cell tray provides more time for root growth before transplanting. It also enables some flexibility in planting time compared to smaller cells if weather causes delays. Cell counts can range anywhere from 28 to 200, with many options in between. The length of time the plants will be in the trays determines how many cells should be in the tray. For growers optimizing space indoors, having trays with a high number of cells could be a great option.

Cell shapes are typically square, round or octagonal. Growers should look for grooves, ribbing or perforations in the sides of trays. These promote downward growth, which provides a good anchor for the plant and is essential in outdoor production. Square cells are the most common across crops that are started in trays. They are easy to find, making them a favorable option. Square and octagonal-shaped cells often have grooves that are optimal for downward root. On the other hand, the walls of round cells are often smooth. This can increase root circling and wrapping rather than downward growth, which can cause problems as the plant grows.

The depth of the cells can also vary among tray types. Deep-root trays are a good option to develop longer roots. Their cells usually provide 5 inches of growing space, compared to 2 to 3 inches of growing space in shallower trays. Hemp grows very fast, so providing extra space for quick root development can keep plants healthy.

Air pruning trays come with a lot of variation as well. The general principle of an air pruning tray is to provide spaces for roots to grow out and down, which allows for a strong root system and does not cause the plants to get rootbound. (Read more about air pruning here.)

Plastic-free options also are available for starting plants. Several companies produce biodegradable pots that can be planted directly into the soil. These are great options for operations trying to reduce their plastic use. They also eliminate the step of removing plants from plastic propagation trays. However, these pots tend to be costlier and need to be repurchased every season, whereas a plastic tray can last at least two seasons.

Outgrowing Propagation Trays

Eventually, plants in trays will need to be moved either to larger containers or out to the field before they become rootbound. The amount of time a plant can stay in a tray will depend on the tray variables described above (in addition to management of temperature, watering, light and fertilization).

Rootbound plants will be more susceptible to diseases and are unlikely to support the plant's weight. This is especially problematic in areas that get a lot of wind or rainfall. When there are periods of excessive rain, water will sit around the twisted roots, providing a good environment for disease or depriving the roots of oxygen. Some plants may still grow large with a confined root system; however, heavy gusts of wind can easily topple these plants.

Root girdling in field-grown hemp

For growers keeping plants indoors, plants will still need to be transplanted to larger containers to account for root growth. If plants stay in small containers, they will be stunted in size. There are many container options to choose from, including different shapes, sizes, and materials. Again, no grow operation is the same, and what works well for one operation may not be favored by another.

In general, growers will opt for pots or bags. Pots have hard sides and are typically made of plastic. They can come in a variety of shapes and sizes, they are easy to sanitize, and they are often inexpensive and widely available. There are also large air pruning pots made of plastic that work the same as air pruning trays. However, pots can be difficult to move with large plants in them.

Grow bags are made of synthetic or natural fibers, come in a variety of sizes and can come with or without handles. Having handles can make moving large plants easier. Many growers like that grow bags are breathable, which can keep roots healthy.

Reusing Trays and Pots

There are risks when reusing trays and pots without proper cleaning. Pathogens, insects and mites can remain on the surface of trays, infecting or infesting future plantings.

The material of the tray, pot or grow bag will determine how it can be sterilized and reused. Growers can use a solution of bleach and soap, followed by a clean water rinse, to sanitize plastic pots. Some fabric growing bags can be sanitized in a washing machine. They can also be washed by hand with a bleach solution. (Biodegradable containers can only be used for one plant and therefore won’t need to be sanitized.)

With so many options out there, it can be difficult to navigate which types of growing containers are going to work best. The selection comes down to the cost and availability of the container, the type of growing methodology, and what kind of materials the grower wants to use.

Marguerite Bolt is the hemp extension specialist at Purdue University’s Department of Agronomy. She received her M.S. in entomology from Purdue University and her B.S. in entomology from Michigan State University. Bolt’s research has focused on hemp-insect interactions and plant chemistry.