The weather this spring has been anything but normal with the up and downs in the temperature as well as the on and off again sunny days followed by many days of cloudy weather. These types of weather patterns can cause problems in transplant production in the greenhouse, especially when it comes time for watering. Hot days may require more than one watering, and cloudy days may require no water. In either case, growers need to anticipate their transplant water needs without over or under estimating watering. Of course, other factors include the growing media and the plant itself. Lets focus on media, for example, growers using a lighter soilless media, these will dry out much quicker than a heavier media and will require more daily watering (e.g., once in AM, afternoon, and evening). Growers using a heavier media may only have to water once a day, or early in the AM and maybe once more in the PM. Either type of media works fine as long as the weather doesn’t change and it fits the growers needs. Proper transplant watering is all about adjustment. If you have stuck to the same daily watering schedule this spring its very likely you have grossly over or under watered your transplants at some point. And, in all likelihood, grossly over or under watered trying to correct the situation. Uniformity is also key when it comes to proper transplant watering. If you find yourself spot watering out of schedule because some flats dry out quicker than others then most likely your uniformity is off. This is easily done when watering is done by hand with a wand. Poor uniformity can also be the result of transplant flats being set on uneven benches or uneven floors. Much like a field, low spots tend to collect the most water. All of this can lead to uneven transplant growth as well as disease and pest pressure, such as fungus gnat problems. If you are experiencing uneven transplant growth, then most likely your watering is uneven. For transplant growth to remain even, flats need to be watered each time to maximum water holding capacity, where each cell in the flat holds the maximum amount of water. An easy way to see this is to watch for water dripping out the bottom of the flat. As important, flats should also uniformly dry out. Below is an example of tomato transplants that have been pulled from the same flat. You can see the differences in plant height as well as root system formation as a result of uneven watering in a “heavy” soilless media. Cells that have remained on the wet side show poor growth and poor root system development because of a waterlogged cell; whereas cells that have been receiving the appropriate amount of water have much better growth and a nice root ball. The weight test. When was the last time you picked up a transplant tray? Picking up a transplant flat every once in a while will give you an idea of how well your watering schedule and uniformity is! Everyone has picked up a tray that has felt like it has the weight of a rock or the tray breaks apart because of the weight, or when you pick it up and it feels as light as a feather. Doing this is a good way to determine if your soilless media is “heavy” – holds more water than you think or if you have been overwatering on days you shouldn’t be or if you have not watered enough. Consistency is key when watering. Going from an extremely wet to an extremely dry transplant tray is not what you want for the transplant production season. You want consistent soil moisture as much as possible, thus proper transplant watering is all about the proper adjustments and knowing your media.

Differences in soil moisture across a single flat of tomato transplants.

Roots of tomato transplants in a flat that has irregular soil moisture.