When to Water Your Greenhouse Crop

In our last blog, we wrote about how easy it is to over water BioStrate Felt. In this blog, we’ll expand on that topic of when to water your greenhouse crop, particularly when you are using Grow-Tech products.


We are often asked how much/often growers should water our FlexiPlugs and BioStrate. While we love to be helpful and wish we could give a simple answer, when it comes to watering in a greenhouse, there isn’t one. Water needs are dependent on a number of factors which vary from one greenhouse to the next. These factors include air temperature, type and size of plant, type of growing media and humidity.

Let’s examine these factors one at a time:

Air temperature: Irrigation level and frequency are dependent on the ambient air temperature. This temperature is dependent on geography, weather, time of year, heating system and quality of the greenhouse. Colder temperatures will slow plant functions. Transpiration will be less so roots will take up less water.

Humidity: The level of humidity in your greenhouse is a function of climate, time of year, the ventilation system, how many plants are in your greenhouse and other factors. Humidity and temperature go hand in hand when it comes to affecting water needs. All else being equal, plants in a high temperature/high humidity environment will need less water or less frequent watering than plants in a high temperature/low humidity environment.

Type & size of plant: Newly emerged seedlings require much less water than mature plants do.

Growing Media: The ability of your growing media to absorb and retain moisture impact how often irrigation is needed. Let’s take a look at Grow-Tech’s media.

  • FlexiPlugs can be used in a traditional greenhouse setting as well as in hydroponics and aquaponics. They are designed to absorb and retain water. If the surrounding air is humid, if the sky is overcast, if the plants are small, less water will be needed. FlexiPlugs arrive already moist. If you squeeze them hard, some water will escape. How much and how often you irrigate after planting is, again, dependent on the stage of crop growth and the crop being grown as well as the temperature and humidity in the greenhouse. In the left hand photo below, the plug is too wet. In the photo on the right, the left hand plug is dried out (note light brown color). The plug on the right is just right. Water will not pool on the top or the plug if it’s squeezed. For a quick demonstration of moistening and seeding this medium visit our Youtube page.

Too wet       2014-12-29 14.03.38

  • BioStrate is designed specifically for hydroponic and aquaponic production of microgreens and salad greens. It is dry so it needs moistening prior to or just after seeding. After that, how much you irrigate will be determined partly by the factors mentioned above and the production system you use.  A sign of overwatering BioStrate is when the felt is covered by water or when water pools on the surface (see photo). BioStrate does well in intermittent watering systems. We were told by one hydroponic grower that one crop of microgreens growing on BioStrate did not receive water for an entire weekend and was just fine. While we don’t recommend that, the story does illustrate that it’s easy to over water.
Too much water.

Too much water.

When watering any type of medium, the plug or pot should be saturated all the way through with no more than 10-15% of the water leaching out. The appearance of water on top of a substrate is a good indication that the medium is over watered. However, if no water leaches out of the container than it’s likely that the water didn’t percolate all the way through which means some roots are left dry. Keep in mind, though, that plants may require less water than you think.

In the end, you are the best judge of when and how much to water. Observation of your plants and the environment in and outside of your greenhouse will tell you how often and how much to water.

Further Reading:

This article from Colorado State University addresses this topic from a conservation perspective.

This article from Greenhouse Grower tells you how to train employees to water effectively.

This article from Michigan State University Extension discusses specific tips for watering more efficiently and effectively.