0524-2808-MTDC: Using a Steamroom To Sterilize Pallets of Styrofoam Seedling Container Blocks: Introduction

Styrofoam container blocks (styroblocks, figure 1) are a popular media for growing seedlings in greenhouse nurseries. They come in a variety of sizes and can be reused several times. They must be sterilized before reuse because they may harbor pathogens that can cause diseases to seedlings. Potential pathogens, such as Fusarium fungi, live on residual organic matter and within the inner cell walls of styroblocks. They also may colonize fragments of roots left on containers after the seedlings have been removed.

[photo] Styrofoam container block (styroblock).

Figure 1—Styrofoam container blocks (styroblocks) are used to grow
seedlings in greenhouse nurseries. The styroblocks must be sterilized after
each use to kill pathogens that damage future seedlings.

Several approaches to sterilizing styroblocks have been investigated. Chemicals such as sodium hypochlorite (bleach) and sodium metabisulfite have been used. But these chemicals present problems with workers' exposure to toxic chemicals and disposal of the chemicals. Many nurseries use hot-water dipping (figure 2), immersing the styroblocks in hot water (from 140 to 160°F) for about 2 minutes. This method kills most pathogens.

[photo] Container blocks about to be submerged in hot water.

Figure 2—Container blocks are loaded in a cage
and submerged in hot water for 2 minutes during
the conventional method of sterilization. A forklift
forces the blocks down into the water.

Hot-water dipping can be time consuming and labor intensive. Styroblocks must be loaded individually into a cage or basket that holds 20 to 50 styroblocks. The cage is submerged in the hot water for 2 minutes. The styroblocks are removed and the process is repeated. At larger nurseries where thousands of styroblocks need to be sterilized, this process can take many days. Additionally, the energy required to keep the water hot can be expensive.

The Missoula Technology and Development Center (MTDC) was asked to look at alternatives to hot-water dipping that would reduce the cost and labor required to sterilize styroblocks.

Highlights...

Styrofoam container blocks used to grow seedlings need to be sterilized before reuse.
The conventional method of dipping the blocks in hot water took workers several days at large nurseries.
Sterilizing pallet loads of blocks in a steam room is effective and more efficient than dipping the blocks in hot water.