Root rots like pythium, phytophthora and rhizoctonia are common diseases that attack young bedding plants in the spring and we often find ourselves treating prophylactically or curatively at some point during the growing season. These diseases have many sources, but plants that are stressed and over-watered are usually more prone to attack.
I recently took an informal poll of growers at a meeting to determine how many growers enjoy the task of “spot-watering” (checking each pot individually and watering according to that individual’s needs). Not surprisingly, no one confessed to liking the job.
“An ounce of prevention is worth a pound of cure” is how the saying goes, and it couldn’t be truer when we consider our greenhouse crops. We often think about preventative strategies like good sanitation and scouting to prevent disease and insect problems. However, the substrate, and how you handle the substrate, is an often-overlooked preventative measure to avoid production and disease problems later in the season. In most cases when we think about disease control and substrates, we want to focus on the physical properties, specifically the pore space and the water-holding capacity of the substrate.
Choose The Right Substrate
Let’s first consider the selection of your substrate. Substrate manufacturers have a multitude of products to offer us; it is our job to select the appropriate substrate for the scenario we are growing in. If we take a closer look at the substrates available to us, we will find dramatic differences between them. The obvious difference is the components that are used to create the substrate. Peat and perlite are the standard components, but we also have vermiculite, coconut coir, compost, rice hulls and bark, just to name a few.
To complicate the issue, each one of these can come in different grades or sizes that will also influence the pore space and water holding capacity. Generally speaking, the larger particle sizes in your substrate will facilitate drainage; therefore the opposite is true, the smaller particle sizes will facilitate water-holding capacity.
We can see this in Figure 1. After irrigation, the water has drained from each of the substrates and only the water that is held on the surface of the particles is left (blue outlines on the particles). The substrate on the left has smaller particle size, and by volume has more water than the larger particles on the right.
How can we use this knowledge to our advantage? Take note of the season, the crop and your irrigation habits. Choose a substrate with better drainage in cooler seasons, for crops that do not tolerate wet conditions, or if you tend to run your crops on the wet side. Conversely, in warmer seasons, crops that require a lot of water or if you tend to run your crops on the dry side, you may choose to use a substrate that has better water-holding capacity.
Different Pot Sizes Drain At Different Rates
The particle size is not the only influence on how our substrates drain. The pots we use can actually have a dramatic effect on substrate drainage. Taller pots drain better than shorter pots.
You can see in Figure 2 the percent of air, water and solids in an identical substrate in different sizes of pots. This phenomenon has more to do with the height of the pot than the diameter. As the pot increases in height, the ability of the substrate within it to hold water against the force of gravity decreases. Therefore, the taller 6-in pot drains better than a 4-inch pot and a plug flat will drain even less, the same way a standard pot will drain better than an azalea pot of the same diameter.
Now you may be thinking about that plug flat that doesn’t drain as well as other taller pots and how often you need to water plug trays, especially as the seedling matures. The rate of drying is not due to drainage, but to evaporation and the water uptake of the plant via transpiration, coupled with the relatively low volume of substrate. So yes, the plug flat might dry out faster, but it isn’t due to the rate of drainage.
Avoid Nesting Pre-Filled Flats And Pots
Many growers prefer to fill flats and pots prior to use. After the containers are filled, they are palletized and stored until they are needed.
When filled containers are filled and stacked, we run the risk of compaction due to pots nesting within each other (Figure 3). When the substrate is compacted, we have decreased or even eliminated the amount of air space in the pot, and air space means drainage. When a grower uses these pots, they will stay wet longer than non-compacted pots and increase the risk of root rot diseases (Figure 4).
The problem is accentuated because the pots on the bottom of the pallet are compacted more than the pots at the top. Once the pots are distributed on the bench, there will be vast differences in drainage and therefore, when they will need to be irrigated again. So begins the endless cycle of spot watering (Figure 5). It is acceptable to pre-fill pots and to stack them, just be sure to offset them or place sheets of plywood between layers to avoid nesting.
Take these simple concepts and apply them to your operation and you will surely be rewarded with healthy plants, a crop with less root disease and fewer frustrations from spot watering.