Watering transplant plugs is more of an art than a science – and good watering (not to mention just keeping the plants alive!) makes a critical difference in the health of your transplants, and ultimately your crops. Unfortunately, it also takes a significant amount of time at a point in the growing season when farmers can least afford it, and the art aspect of the job makes it hard to delegate. But it can be done.
When Rock Spring Farm made the transition from soil blocks to plug trays, we faced a significant challenge. We made the change because, even with the mechanical block maker that we had used for years, soil blocks required so much labor and care both when we made them and when it was time to set them out in the field. Because of the relatively large amount of soil in each block, and their ability to share water between blocks, soil blocks grew great transplants without much attention. While they were easier to fill and set out in the field, plug trays required a great deal more care as we grew them, particularly when it came to watering.
When Rock Spring Farm made the transition from soil blocks to plug trays, we faced a significant challenge. We made the change because, even with the mechanical block maker that we had used for years, soil blocks required so much labor and care both when we made them and when it was time to set them out in the field. Because of the relatively large amount of soil in each block, and their ability to share water between blocks, soil blocks grew great transplants without much attention. While they were easier to fill and set out in the field, plug trays required a great deal more care as we grew them, particularly when it came to watering.
When to Water
I like to water at mid-morning. By then, the sun is up in the sky and the greenhouse has warmed up enough that the water you put on the leaves of the plants will dry quickly. If you leave the plants wet overnight, you increase the potential for fungal germination and bacterial propagation.
Like most things that matter on the market farm, good plug tray watering takes management – you can’t treat every tray in the greenhouse the same, and every day is different. Shorter days, cloudier weather, high humidity, and lower temperatures will result in plants that require less water. In addition, the maturity of the plant can make a dramatic difference in water needs. It’s easy to overwater young plants, and just as easy to underwater mature plugs.
Location in the greenhouse can also cause variation in how quickly plants use available water. Areas with lots of air movement will dry out faster, and can result in significant variation even in adjoining flats. Bench edges often dry faster than centers.
You shouldn’t assess water needs by how the plant looks – you want to anticipate drought stress, rather than respond to it. Because water is heavy, lifting the trays can give you an idea of water left in the potting medium. Pull plants out of their cells, or look at the bottom of the tray, to see if the soil mix at the bottom of the cell has moisture in it. If you squeeze the root ball, a little water should drain out of it.
Like most things that matter on the market farm, good plug tray watering takes management – you can’t treat every tray in the greenhouse the same, and every day is different. Shorter days, cloudier weather, high humidity, and lower temperatures will result in plants that require less water. In addition, the maturity of the plant can make a dramatic difference in water needs. It’s easy to overwater young plants, and just as easy to underwater mature plugs.
Location in the greenhouse can also cause variation in how quickly plants use available water. Areas with lots of air movement will dry out faster, and can result in significant variation even in adjoining flats. Bench edges often dry faster than centers.
You shouldn’t assess water needs by how the plant looks – you want to anticipate drought stress, rather than respond to it. Because water is heavy, lifting the trays can give you an idea of water left in the potting medium. Pull plants out of their cells, or look at the bottom of the tray, to see if the soil mix at the bottom of the cell has moisture in it. If you squeeze the root ball, a little water should drain out of it.
How Much to Water
In the small space of a plug tray, the air space in the maintained more by how much water is added than by the particle size of the root medium. Unlike for large, potted plants, leaching is not recommended unless salt levels are excessive – and that won’t be likely in an organic system.
Don’t rely on your eyes. You can’t accurately assess the moisture in the cells just by examining the top of the cell. More than once, I’ve pulled plugs out of trays only to find that the bottom of the cell was bone dry and completely hydrophobic; the only way to rewet it was to set the tray in a tank of water.
In deciding how much to water, ask yourself a few questions: Based on the current size of this crop, how fast will it use the water that I apply? How big is the cell relative to the plant? What does the weather look like for the rest of the day? Regardless of the conditions, the goal is to adequately saturate the root media to sustain the plant to its next irrigation.
For shallow plug trays, you don’t need to fill the soil to capacity in order to have even water distribution throughout an individual cell. Consistent overwatering leads to waterlogging and nutrient leaching, and should be avoided, unless you have a need to flush the salts out of the planting mix; this shouldn’t be a concern in most organic fertility systems.
Don’t rely on your eyes. You can’t accurately assess the moisture in the cells just by examining the top of the cell. More than once, I’ve pulled plugs out of trays only to find that the bottom of the cell was bone dry and completely hydrophobic; the only way to rewet it was to set the tray in a tank of water.
In deciding how much to water, ask yourself a few questions: Based on the current size of this crop, how fast will it use the water that I apply? How big is the cell relative to the plant? What does the weather look like for the rest of the day? Regardless of the conditions, the goal is to adequately saturate the root media to sustain the plant to its next irrigation.
For shallow plug trays, you don’t need to fill the soil to capacity in order to have even water distribution throughout an individual cell. Consistent overwatering leads to waterlogging and nutrient leaching, and should be avoided, unless you have a need to flush the salts out of the planting mix; this shouldn’t be a concern in most organic fertility systems.
How to Hand Water
For most small farms, hand watering is the obvious choice. Choose an appropriate nozzle – I recommend a fan-shaped nozzle rather than a round one, as the rectangular pattern creates more consistent results than a round one.
Adjust the water pressure just enough so that the individual streams of water from each hole hold together nicely. Too much pressure is unnecessary and can wash seeds and smaller plants; too little pressure can result in large, blobby streams of water with much the same results. I really like to have a quarter-turn ball valve on each watering wand to allow the user to adjust the water as she goes, to account for changes in water pressure over the course of the job.
Start by spot watering, bringing every plug in the greenhouse up to a similar water level.
Then, holding the fan-shaped nozzle so that it is pointed straight at the crop, walk the full length of the bench holding the wand at a consistent height above the crop. Go just past the end of the bench, then turn around and take another pass on the next set of cells that didn’t receive water on the first pass.
Multiple light passes can help to achieve more consistent results. Also, you might want to offset any behavioral differences that occur between the beginning and end of your watering sessions by starting at a different end of the bench each time; or work from the edges in one day, and from the centers out the next.
Start by spot watering, bringing every plug in the greenhouse up to a similar water level.
Then, holding the fan-shaped nozzle so that it is pointed straight at the crop, walk the full length of the bench holding the wand at a consistent height above the crop. Go just past the end of the bench, then turn around and take another pass on the next set of cells that didn’t receive water on the first pass.
Multiple light passes can help to achieve more consistent results. Also, you might want to offset any behavioral differences that occur between the beginning and end of your watering sessions by starting at a different end of the bench each time; or work from the edges in one day, and from the centers out the next.
Automatic Watering
If you go with an automatic watering system, you will need to add more management and observation to accommodate the automation. I have worked with more than one transplant operations that installed automatic watering systems to save labor, and promptly drowned their plants.
Make certain that your sprinkler system lines up with your benches – this sounds obvious, but even the best-designed system will fail if it’s not installed correctly.
Once again, spot-watering is a key element to automatic watering if you don’t want to dramatically overwater or underwater some trays.
Keep an automatic watering log to provide an ongoing feedback tool. Note the time of day, weather conditions, and how long you ran the system on each bench each time you run the system. Over time, this can help to develop a feel for how much and how frequently to water. At Rock Spring Farm, we ran the water once in the morning, and checked again at mid-afternoon to assess whether we needed to water again.
Make certain that your sprinkler system lines up with your benches – this sounds obvious, but even the best-designed system will fail if it’s not installed correctly.
Once again, spot-watering is a key element to automatic watering if you don’t want to dramatically overwater or underwater some trays.
Keep an automatic watering log to provide an ongoing feedback tool. Note the time of day, weather conditions, and how long you ran the system on each bench each time you run the system. Over time, this can help to develop a feel for how much and how frequently to water. At Rock Spring Farm, we ran the water once in the morning, and checked again at mid-afternoon to assess whether we needed to water again.
Managing Employees
Watering plug trays isn’t a job for just anybody on your crew. It takes sensitivity and a will to excellence to do a good job at this critical task. But it can be delegated if you create a reliable system. I recommend limiting watering to one primary and one secondary staff person. And an individual will gain a sense for the variations in water needs according to location and growth stage that is hard to communicate – it is an art, after all.
If you choose to delegate this task, you’ll want to have a plan for monitoring and oversight on a regular basis. Take the time to walk the house on your weekly field walks – this can be a great way to institute this critical success habit early in the season, before you get to the field. You might even want to do this if you water your transplants yourself.
If you choose to delegate this task, you’ll want to have a plan for monitoring and oversight on a regular basis. Take the time to walk the house on your weekly field walks – this can be a great way to institute this critical success habit early in the season, before you get to the field. You might even want to do this if you water your transplants yourself.