Fig. 1. Substrate-filled containers stacked or “nested” on top of each other resulting in substrate compaction.
Photo: W. Garrett Owen
Fig. 2. Liner trays (A), pots (B), and hanging baskets (C) prefilled with substrate and stacked or “nested” on top of each other and placed on pallets or carts. Substrate compaction occurs.
Photo: W. Garrett Owen

January. The beginning of a new year and for most, the start of another growing season. During this time, most growers are preparing for the arrival of annual and perennial unrooted cuttings, plugs or liners. Preparation may consist of a neverending “to do” list, but one thing is certain: Pre-filling liner or plug trays, pots, flats, hanging baskets, and any other growing container has likely made the list. Pre-filling containers with substrate is a common production practice I have seen among growers worldwide to reduce labor costs, but most importantly, to save production time during the busiest spring months. However, more times than not, I come across substrate-filled containers that are stacked or “nested” on top of each other (Fig. 1) and to a lesser extent, wrapped or covered in plastic.

Fig. 3. Substrate compaction occurs from the weight of the containers nesting within each other.
Photo: W. Garrett Owen

I have seen plug and liner trays (Fig. 2A), pots (Fig. 2B), and hanging baskets (Fig. 2C) prefilled with substrate and stacked or “nested” on top of each other and placed on pallets or carts. Stacks of containers are sometimes covered with plastic and placed under either protection or stored outdoors under ambient sunlight or covered in snow. I have even seen where plastic wrapped pallets were stacked on top of each other causing considerable weight pressing down on substrate-filled pots. Additionally, in some instances, I have witnessed plug trays intentionally being compacted into each other resulting in a dibble to make seed sowing easier. Regardless, when containers are filled with substrate and stacked, substrate compaction occurs from the weight of the containers nesting within each other (Fig. 3).

Fig. 4. To prevent substrate compaction, place rigid materials such as polycarbonate sheets (A), plywood (B), or metal shelves (C) between layers of substrate-filled containers.
Photo: W. Garrett Owen

You may be thinking, “So what if I handle by substrate in this manner?” or “What is the big deal with stacking and nesting prefilled containers?” When substrate is compacted, it often hinders the roots ability to displace substrate particles necessary for proper growth and development. This resistance or restriction is termed mechanical impedance. The restriction for root growth and development also affects the plants ability to supports itself in containers, one of the four functions of a horticultural substrate. Compacting the substrate decreases total porosity (pores) and air space (aeration) by 50 percent or more, causing poor drainage. Substrates that drain poorly can become waterlogged and increase the likelihood of a root rot disease to occur. Research at the University of Florida has indicated that as compaction increases, there is a higher “dry bulk density” or the ratio of the mass of dry solids to the bulk volume of substrate in the containers. Consequently, this reduces total porosity and air space.

Dr. Roberto Lopez of Michigan State University and I conducted an experiment to determine how substrate compaction influences shoot and root growth and development of bedding plants grown in containers. We prefilled 4.5” pots to the rim with a commercial soilless peat-based substrate and placed pots in shuttle trays with a carrying capacity of 15 containers per tray. We determined the weight of a single container-filled shuttle tray and the weight of each subsequent tray nested on top of another until a total of nine trays were stacked. Stacked trays were covered with 4-mil black plastic and placed on a greenhouse bench for a week to simulate storage. We measured substrate compaction and investigated the growth effects of African marigold, pansy, and petunia seedlings transplanted into five containers from each layer of stacked shuttle trays. Essentially, we found that substrate compaction increased as the layer of shuttle trays increased from 4.8 pounds (second layer from top) to 38.3 pounds (ninth layer or bottom). Root dry mass was negatively affected by substrate compaction and declined significantly from the fifth to the bottom shuttle tray layer. However, vegetative growth did not appear to be as negatively affected by substrate compaction. For more information about this experiment, refer to the e-GRO Alert, “Avoiding container substrate compaction” (bit.ly/2nQDHSH). Based on the results of this experiment, it is recommended to take action and prevent substrate compaction.

Fig. 5. To prevent substrate compaction of plug or liner trays, simply offset or stagger the trays where the cells do not nest in one another
Photo: W. Garrett Owen

How do you prevent or alleviate substrate compaction from occurring and still pre-fill your containers? There are a few options depending on the container. For plug or liner trays, you can simply offset or stagger the trays where the cells do not “nest” in one another (Fig. 5). This option will also work well for containers. Now, say you want to fill and store the containers. The best option is to place rigid materials between each layer of trays, flats, pots, or hanging baskets. I have seen rigid materials from disassembled young plant shipping boxes, cardboard, polycarbonate sheets (Fig. 4A), plywood (Fig. 4B), and metal shelves (Fig. 4C). You can also prevent substrate compaction by placing your flats or pots in trays with webbed or solid bottoms (Fig. 6). It is recommended to stack these trays where the corners do not compress the substrate.

Fig. 6. Substrate compaction can be prevented by placing pots in trays with webbed or solid bottoms.
Photo: W. Garrett Owen

Overall, take the time to place rigid materials between layers of substrate-filled containers. It will prevent any compaction from occurring and alleviate restricting root growth and development. Take action and prevent substrate compaction!

Garrett is a Floriculture and Greenhouse Outreach Specialist at Michigan State University. wgowen@msu.edu