Essential Equipment

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If you think about it, there has been a huge increase in tools and instruments available to greenhouse growers during the last couple of decades that vary from large instruments like greenhouse control computers, to more portable instruments that help with day-to-day monitoring of plant growth. This article outlines some of the portable instruments available today and advantages and disadvantages of each.

In many cases, these instruments can save you money by avoiding production problems, reducing the amount of outside testing that needs to be done and helping in immediate problem solving. How many times would you like to know your pH and EC immediately to decide what to fertilize with at that moment? What is your plant temperature? Is your media temperature what you think it is? New instruments available today can answer these questions. 

Infrared Thermometers

Temperatures are rarely what we think they are! This is especially the case for the surface temperature of plants and the media we grow our crops in. It is amazing how often a grower thinks they know their media temperature in a plug tray or rooting liner, when, in fact, the media has a very different temperature. I have been in greenhouses where the plug media temperature was 95˚F when a grower thought it was 72˚F. In contrast, I have been to vegetative propagators’ facilities where a grower thought their rooting media was 72 to 74˚F, when in fact it was 60 to 63˚F. In each case, germination and rooting, respectively, were greatly reduced.

How could they have avoided this? By purchasing and using an infrared thermometer. Infrared thermometers measure the surface temperature of whatever you point them at. This piece of equipment has one of the quickest paybacks of any that you could purchase. They help you reduce heating costs when plants are warmer than you think and help improve germination and rooting by providing optimal temperatures. All greenhouse temperatures should be based on the plant and media temperatures – not air. This instrument helps you to do that.

An infrared thermometer measures temperature by measuring the infrared heat an object is giving off. Usually, infrared thermometers also come equipped with a laser built into them, so you can see exactly what you are measuring; usually with a lighted red dot that illuminates where you point the device. This is especially important because the thermometer measures the temperature in a relatively small area and you want to know where that is! You can see more information on infrared thermometers at

In many cases, infrared thermometers must be within 2 feet of the object they are measuring to be accurate. As you move farther away, the resolution decreases because they are measuring a much broader area than what you think. Just because you can see the laser 10 feet from an object, does not mean that the thermometer is accurately measuring the temperature at that point accurately!

Also, infrared thermometers vary in their resolution or how close to the actual temperature they will be. Some vary +1˚F, others are +3˚F. The more resolution you want, the more expensive the thermometer.

In general, prices for infrared thermometers vary from $50 to $300. The most expensive thermometers will have the highest resolution (+1˚F), whereas, the cheapest will have the least resolution (+>2˚F). 

pH Meters

The pH impacts whether nutrients in the media are available for plant growth or are tied up in the media. Many of the crops we grow today have very specific pH requirements. For instance, we know that geraniums and New Guinea impatiens should not be grown at pH levels below 5.8. In contrast, petunias, scaveolas, calibrachoas and nemesias should not be grown at pH levels above 6.5. If pH levels drift out of the optimal range, plant growth is reduced and nutrient toxicity and/or deficiencies will occur. It is becoming increasingly important to monitor pH on a regular basis, because pH in soilless media can change rapidly, and because we have crops that vary a lot in optimal medium pH levels now.

Meters that measure pH have been available for years, however, they have been expensive and notoriously variable or unreliable. Today, they are cheaper and smaller but can still have reliability problems. Regardless, they are well worth having and are very helpful in growing crops.

It is very important to protect the reference standard in the probe! The most common problem with pH meters is that people are rough with the probe and break it or they do not keep the probe in a solution when not in use (which is often required). Some new meters measure pH by inserting the probe directly into the media. To date, I have not seen that the accuracy/dependability of these equals that of the traditional meter.

Ideally, most of us try to grow crops with a media pH from 5.8 to 6.4. A pH of 6.2 to 6.4 is the best. Meters differ in their accuracy and resolution. Accuracy refers to how precisely the meter will measure the pH of the solution. Resolution will tell you how many digits will be reported after the decimal. Choose meters with accuracies of +0.02 and resolutions of 0.01 pH units. Additional reference material can be found at:

Solution temperature is important unless you have a temperature compensated meter (ATC). If you don’t, the temperature of the water solution that you are measuring should ideally be 77˚F. Meters are calibrated using two calibration solutions – one 4.0 and one 7.0. Of all the meters here, a pH meter tends to drift more and requires regular calibration. I have found that the surface sensor monitors can break easily, and the best meters are still the lab-grade meters that sit on a bench and you bring the sample to them. 

Soluble Salts Or Electrical Conductivity (EC) Meters:

EC or soluble salt meters measure how much salt is in the media. Why is this important? Too much media salt will result in water actually leaving the roots and moving into the media. This will result in root burning and eventually result in root death and rot.

Aside from measuring when salt levels are too high, EC meters are great ways to determine if your injector is working the way you think it is. If you know the EC of your water, and how much EC you are adding via fertilizer injection, you can determine whether your injector is under or over injecting.

We measure conductivity in the greenhouse industries with a number of units including micromhos (umho), millimhos (mmhos), microsiemens (uS), millisiemens (mS) and decisiemens (dS). (1000 umho/cm = 1000 uS/cm = 1 mmho/cm = 1 mS/c = 1 dS/m)

The standard we use in the greenhouse industry today is mS/cm and the range is typically from 0 to 5.0. An EC reading without a unit attached to it is usually in uS/cm. Therefore, divide this number by 1000 to get mS/cm. In general, media EC should be maintained between 0 and 3.0; as EC increases above 3.0, root burning increases.

As salt levels in a media/solution increase, the media/solution can conduct more electricity. A soluble salt meter has two probes, and measures how easily electricity is conducted between those two probes when dipped in a solution. Media with high salts conducts electricity easily. In contrast, media with low salts conducts electricity poorly. Therefore, distilled water (no salts) is a poor conductor (usually zero). In contrast, a solution with 600 ppm N from fertilizer is a great conductor.

EC/Soluble salts meters are inexpensive and very reliable. EC meters come in three different uS measurement ranges: Low (0-199 uS), medium (0-1999 uS) and high (0-5.0 mS or 0-19.99 mS). In general, most greenhouse companies should purchase the one that is in the high range (0–5 mS or 0-19.99 mS). Also, you should purchase a meter with accuracies of + 0.02 and resolutions of 0.01 mS. Calibrate EC meters using a single standardized solution that you can buy premade, or make yourself using distilled water (follow directions) and mixing in a packet that you purchase. Regardless, choose a standard that is near 1.41 mS as that is a reading that is similar to what you will be measuring in greenhouse media.

John Erwin is a professor in the Department of Horticultural Science at the University of Minnesota. He can be reached at

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