This post is going to cover everything you need to know about fertilizer, water, and watering schedule. My intention is to be as thorough as possible, so feel free to scroll through and read the parts that you're interested in.
When it comes to online information about growing houseplants, vague terms abound. For example, many online sources talk about using soft water on tropical plants or grow plants in bright diffused light. While not inherently incorrect, these recommendations are not very helpful and can even be misleading because it doesn't tell you exactly how hard or how bright Now, I am not saying that these terms are wrong, I'm just saying that these terms aren't very helpful. They don't give us specific information, so they can be confusing. The way to eliminate some of the confusion is to be specific and measure things.
When growing plants in a living wall set up it's important to measure your water quality, your nutrient solution concentration, and the nutrient solution pH. Measuring these parameters will actually make it easier for you to ensure your plants stay healthy and it will help you prevent a lot of potentials issues.
Key Measurements
TDS - or Total Dissolved Solids
Measures the amount of dissolved minerals in your water. You can use it to measure how soft or hard your water is, and how "strong" your fertilizer solution is. The only way to do it is to boil the water and measure the remaining minerals. Since it would be cumbersome to do it that way, you can get a ppm meter that measures EC (electrical conductivity) of the water and converts it to TDS; TDS is measured in Parts Per Million (ppm). There are two different scales for measuring ppm (500 scale and 700 scale). In the US we use the 500 scale for measuring water TDS. Here is an article to learn more: different PPM scales explained. It might also be useful to understand Electrical Conductivity in Hydroponics.
I bought a meter that can measure both TDS and EC, but I usually only measure TDS. You can buy a TDS meter on Amazon for under $50.
pH
is the measure of how acidic or alkaline a solution is. The scale is from 0-14. pH 7 is neutral. pH values less than 7 are acidic and greater than 7 are alkaline. You can read more about pH in hydroponics here: pH in hydroponics
pH is important because it affects the plant ability to absorb nutrients. When growing plants in soil the ideal pH range is between 6.5-7. In soilless hydroponic solutions (in a living wall), the pH has to be between 5.6-6
pH is best measured by using a pH meter. The meter has to be calibrated using a calibration solution, and the probe has to be kept wet using a special storage solution. You have to calibrate the pH meter every time you make an accurate measurement. Most of the cheap meters available online are inaccurate. I have been using a Blue Lab pH meter. Blue Lab meters are more expensive but are very reliable and accurate. You can purchase a pH meter on Amazon or at a local hydroponics store.
It is important to measure your pH. Adjusting it without properly measuring it will kill your plants very quickly. The pH of your tap water can change. You can buy a quality pH meter for about $50 on Amazon or your local hydroponics store.
Water Quality
Hard water contains high percentages of calcium and magnesium, and has a higher TDS. According to this article, soft water is from 0 to 60 ppm. 61 to 120 ppm is moderately soft and anything above 120 is hard. (note that the article uses mg/l not ppm. 1/mg L is approximately 1 ppm) My tap water is at about 300ppm, which is not ideal. Many tropical plants, especially ones that come from higher elevations, are particularly sensitive to a high mineral content.
If the TDS of your tap water is greater than 100 ppm, you should consider using a reverse osmosis filtration system.
A reverse osmosis filtration system uses a semipermeable membrane to filter out most of the dissolved minerals. You can buy an RO filter on Amazon for about $70. You will have to connect it to the water supply underneath the sink.
Alternatively, many grocery stores have a drinking water dispenser where you can fill your water jugs and pay at the register. That water is actually RO and works well. It costs me about $1.50 to refill a 5 gallon jug. I've measured the water at several nearby grocery stores and the water is usually around 40 ppm, which is perfect for plants.
If neither of the options are available to you, then use your tap water and see how your plants do. What you should avoid at all costs is softened water which is toxic to plants. You can read more about softened water here water softeners explained
Fertilizer
In a living wall setup the plants grow without any soil, so it is important to have all of the necessary nutrients dissolved in the water.
There are many great fertilizers out there, but because I haven't tried them on the wall, I cannot recommend them. When trying a new fertilizer, be cautious, take notes, and remember that less fertilizer is always better in the beginning.
It is best to use a hydroponics fertilizer, because hydroponic fertilizers are designed to remain stable in solutions, and contain all necessary elements.
When it comes to concentrations, always make it weaker at first, because the concentrations suggested on the bottle were designed with commercial crop production in mind. While some plants, such as philodendrons, might do well with those concentrations, more delicate plants may not. It's best to have a concentration that works for all of your plants.
Additionally, a stronger concentration might work at first, but might result in stunted growth in a few weeks due to salt build up and root burn.
Many fertilizers have a high phosphorus (P) content which is represented by the middle number. For instance, in a 9-3-6 fertilizer 3 represents Phosphorus. It is best to look for fertilizers where the ratio of the first to the second number is roughly 3:1. Such as 3-1-2 or 9-3-6. Overall, you don't need a high phosphorus ratio to induce blooming, as many of my plants bloom several times per year even though I use a low phosphorus fertilizer.
Fertilizer I have used on my wall
Dyna Gro Foliage Pro
A great fertilizer that is very popular. When using RO water with it, it is necessary to use a Calcium and Magnesium supplement because it doesn't have enough of those elements.
Although the bottle suggests that you can use as much as 2 tsp per gallon, in my experience, it can burn roots at a concentration higher than 1 tsp per gallon. With that being said; my plants, especially Begonias, Philodendron Melanochrysum, and Amydrium Zippelianum were growing best when I used foliage Pro at 1 tsp per gallon. They suggest a maintenance dose of ¼ tsp per gallon. The fertilizer is prone to precipitation if stored in cold temperatures. When buying it, shake it and listen to hear if something is moving inside. It means that some of the components have precipitated and you should buy a different bottle. If you find precipitation in the bottle, consider emailing the company and they might replace it for free.
Cyco Grow Platinum Series A and B
Cyco Grow is a great fertilizer that comes in two bottles - part A and part B. My plants have done very well. Their claim to fame is that the elements in the fertilizer are chelated and are easier absorbed by the plants.
One of the downsides is that you need to use more of it per gallon than Dyna Gro. The optimum dose for my plants is around 6-7 ml per gallon, as opposed to 4.9 ml/gal of foliage pro.
Like with many liquid fertilizers, precipitation can be an issue. If you notice that your fertilizer bottles have precipitation in them, email the company and they should replace it for free.
Disadvantages: the fertilizer is not as pH stable as some other fertilizers.
Cal Mag Plus
When using RO water, it is important to use a calcium and magnesium supplement, because most fertilizers aren't designed to be used with RO water.
I use it at the strength of about 2 ml per gallon. It is important to add this product first and adjust the pH to about 6. before adding the main fertilizer.
There are other fertilizers that might work well or even better than what I've used.
pH Adjustment
Although some people suggest using citric acid or baking soda for pH adjustment, these products have only a temporary effect on the nutrient solution, and tend to be ineffective to maintain a certain pH over a long period of time. I would suggest using products from the Hydroponics industry such as pH Up and Down. (you usually use one or the other depending on what your current pH is ) Whenever adjusting the pH, use it sparingly. most of the time, you only need a few drops per gallon. If you need to adjust the pH of a solution in a reservoir with the fertilizer already mixed in, dissolve a few drops of pH Up or Down in a small amount of water, then mix it into the nutrient solution. Measure the pH while doing this, then wait 30 minutes and measure it again.
Until you get a feel for your system, measure the pH daily.
It is important to always, always measure your pH when adjusting it. If you try to adjust it without measuring it, you will kill your plants very quickly. Some people have asked me how much pH Up or Down I use so that they could do the same in their setup. The truth is that it changes every time, and if I wasn't measuring it, I wouldn't have been able to have the pH in the favorable range.
Preparing the Nutrient solution
Although I mentioned the steps previously, I will briefly outline them here to eliminate any confusion
1) measure the pH and TDS of Water
2) using a graduated cylinder or a measuring cup measures the required amount of Cal-Mag and and it to the water. Mix it thoroughly.
3) measure the pH
4) add enough pH up or Down to bring the pH to about 6.5-7 (you want your pH to be a little higher at this stage because the fertilizer will make the water more acidic. For me if I adjust the Ph to about 6.6 at this stage, then when I add the fertilizer the pH is right where I want it to be)
5) measure the amount of fertilizer and add it to the water. Mix it thoroughly. Measure the pH again, adjust it if necessary.
6) measure the pH in 30 minutes and adjust it if necessary.
After a little bit of experimentation I determined that with Dyna Grow the concentration of 400 total (using RO water ) seems to work best. Similarly, with Cyco Grow and Cal Mag, the plants like it best when the nutrient solution is about 400-420 ppm
Topping off the reservoir
As the water is used by the plants, or evaporating over time, the nutrient solution will become more concentrated. It is a good practice to measure TDS and top off the reservoir when the level of water decreases and the TDS rises. Eventually, you'll want to replace the nutrient solution. I usually do it about once or twice a month.
Replacing Nutrient Solution
Cyco Grow suggests completely replacing the nutrient solution every week. I don't actually do that. I top off the reservoir for the first two weeks. After two weeks, if the PPM drops below what it should be, I might add a few gallons of nutrient solution. I've only been replacing my nutrient solution about once a month. It is ultimately up to you. As you watch your plants grow, you'll be able to figure out what works best. Just be aware of the fact that the plants use up some of the nutrients in the solution. The elements might lock up or break down and become unavailable to plants.
When replacing the solution, it is best to use a mild bleach solution to wash the reservoir. Make sure you flush it thoroughly when you do. Additionally, you can fill it up with water, add some hydroponic enzymes, and run the pump for about 15-30 minutes. Then, empty the reservoir and fill it with fresh fertilizer solution.
Some enzyme products are compatible with fertilizers and can be used with the fertilizer at all times. they will help you keep the algae and bad bacteria at bay. I would suggest using the same brand of enzymes as the fertilizer to ensure compatibility. I have found that Dyna Gro Zyme works with Foliage Pro, but might be incompatible with Cyco Grow. However, Cyco Grow has its own enzyme supplement.
Nutrient Solution temperature
In addition to pH, another important factor that affects the ability of plants to absorb nutrients is temperature. Tropical plants tend to be particularly sensitive to lower temperatures of the nutrient solution. A simple way to improve that is to place a small Aquarium heater and set it to about 70 degrees.
Watering Regimen
Roots need air, nutrients, and water. Although it is common knowledge that roots need water and nutrients, we tend to forget that roots need air as well. Too much water means not enough air. When roots are suffocated, roots can start rotting. The key is to allow the roots to dry out. The tricky part is figuring out how much. Some plants need to get bone dry between waterings, while others merely damp. In my experience, it seems that my anthuriums like to dry out more than my philodendrons so I plant my philodendrons to stay damper than my anthuriums. Ultimately, you will start noticing these things about your plants and can adjust your watering schedule accordingly.
When the plants are first planted and need to be established, you might water more often to help them combat the dry air in the room. However, as they get established, consider reducing it. Two layers of felt are utilized in the design, the bottom layer tends to be damp, even when the top layer is dry to the touch.
For the last three months I've had it set to 15 minutes every 6-9 hours. Except at night, I would let it sit for about 12 hours at night. However, recently, as it has been getting warmer and I have increased the watering schedule during the warmest hours of the day. I might increase it even more in the summer.
The pump is usually on for 15 minutes at a time, which works great at flushing the roots and prevents damage caused by mineral buildup.
It is best not to have the pump on after the lights go off, because it creates a perfect environment for mold. In my case, my wall is next to a window. In the winter months it gets cool at night due to a draft in the front window. If my night temperatures are warmer, I might have to have the pump turn on at night.
Conclusion
Starting your Green Wall can be a bit scary, as some of the plants might be expensive or have a lot of sentimental value to you. But gradually you will figure out how to take care of your Living Wall in a way that enables your plants to thrive. The content in this article is meant to give you a basic understanding of what your plants needs are, so that you can figure out what would work best for your particular set up. Although you might be nervous about your Living Wall at first, remember that even some of the most delicate plants are a lot more resilient than what most people think.
In the next post I will focus on plant diseases and how to ensure that your plant wall is disease free. In particular, I will focus on what to do BEFORE installing your plants into the wall, as well as some mistakes that I made when putting plants into the wall.
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