Written by: Jenelle Schafer
What is fertilizer and why do plants need it?
Fertilizer is the “food”, or nutrients, plants need to in order to function properly, grow larger, and bloom. In a wild environment, plants get nutrients in the soil and these elements are continually renewed through many different natural processes, such as microbial activity and leaf litter decomposition. However, when plants are raised in containers and indoors, their access to nutrients is entirely dependent on their cultivator as all nutrients in a pot will either be taken up by the plant roots or leached out during waterings. And so, the restoration of nutrients is done through fertilization.
There are certain elements plants need to function properly and the three that they need in the largest amount are Nitrogen, Phosphorus, and Potassium. Nitrogen is needed to give the plant green leafy growth, phosphorus is used for root establishment, and potassium acts as a “vitamin” and aides in many different plant functions, including flowering. While these three are needed in the largest volume, plants also have other elements that are commonly provided by fertilizer that are necessary for them to grow, but they only need them is smaller quantities. These other elements include Ca, Mg, S, Fe, Mn, B, Cu, Zn, Mo, Cl, and Ni.
When plants are in need of these nutrients, they will often let you know. Some symptoms of nitrogen deficiency are yellowing of older leaves and slowed and spindly growth. Phosphorus deficiency will present as a purplish coloring of older leaves while potassium deficiency will cause leaf tips to brown and leaf margins (leaf edges) to yellow. If you see any of these symptoms in your houseplant and you know the plant has not been enduring any water stress, then you can tell your plant is in need of some nutrients that they cannot access from the potting soil.
When should houseplants be fertilized?
During a growing year, houseplants should be fertilized from March through September. These are months where natural daylight periods are longer, and plants do most of their growth within this window. From October to February, houseplants should be rested. This means watering frequency should decrease slightly and the plant shouldn’t be given fertilizer so that, while the plant has less access to natural light, it doesn’t get stressed to keep up the same pace of growth and flowering that it does the other months.
Now within the months in which houseplants are fertilized, the frequency at which the plant should be fertilized will vary from species to species. I generally classify my houseplants into three categories of fertilizing frequencies: frequent, infrequent, and exceptions. Frequent feeders are plants that should be fertilized twice a month, where infrequent feeders are fertilized only once a month. The exceptions category is reserved for plants that either only need to be fertilized once or twice a growing season because they have a slow growth rate or sometimes you have a plant that grows more like a weed that a houseplant, in which case I never fertilize it unless I see specific symptoms of nutrient deficiency. When determining which category a plat should fit into, I look for key words and phrases while researching the plant. Frequent feeders are commonly be referred to as heavy feeders or sources may say they need to be fertilized often or other wordings indicating the plant will need a high amount of nutrients to thrive. Infrequent feeders are often called low or slow feeders and research will likely direct you to fertilize them monthly.
What fertilizer should be used?
Fertilizers will list the percentage of nitrogen, phosphorous— listed as phosphate, P2O5— and potassium— listed as potash, K2O. Generally, you will find these numbers in this order with dashes separating them prominently somewhere on the label, but in some cases, they may just be listed on a chart with other micronutrients and trace elements. These numbers can vary depending on brand or formulation, but my recommendation would be to find a general purpose plant fertilizer with a 10-10-10 or a 20-20-20 NPK ratio. If you cannot find one of these, and the NPK numbers are all different, for houseplants, I would look for a higher N percentage, because that is what the plant will need to most of to look green and flourishing above the pot and as long as the fertilizer has P and K percentages, you should be fine.
Another thing you will encounter with fertilizers are slow-release and water-soluble or granular fertilizers. Water-soluble fertilizers are mixed with water and applied through normal watering practices. Nitrogen, especially, that is in a form that makes it water-soluble is in the easiest form for plants to be able to access it and use it right away. The downside of water-soluble fertilizers is that every time the plant is watered, those nutrients still in the potting soil will leach and drain out every time the plant is watered. But, with a scheduled fertilization plan, this doesn’t become too big of an issue, and it ensures that certain nutrients don’t build up in the soil to a toxic level. Slow-release fertilizers are made as beaded capsules or sticks that are sprinkled on the top of potting soil. Every time you water, a little bit of the fertilizer dissolves and those nutrients become unlocked to the plant. The problem with slow-release fertilizers is that it is hard to control how much fertilizer the plant is receiving at any given moment. Slow-release fertilizers are a better option for plants in large containers outside or planted in the ground because there is much more water flowing through those soils and unlocking more nutrients on a more consistent basis than in a small pot that is only watered every 7-10 days. So, for your houseplants, I would recommend that you stick with water-soluble fertilizers.
Probably the most common brand of fertilizer for houseplants is Miracle Gro. Miracle Gro’s Water-Soluble All Purpose Plant Food has a 24-8-16 NPK ratio and can be found easily at practically any hardware or large box store. The slight downside of Miracle Gro is it often contains a lot of elements in a salt form and after a lot of applications, these salts can build up on pots and even plant stems and leaves. Another brand that is pretty common is Espoma. Espoma is an organic company that has a large line of slow-release fertilizers, but they do also have one called Indoor! Houseplant Food that is a water-soluble fertilizer with a 2-2-2 formulation. My favorite fertilizer is Jack’s Classic All Purpose 20-20-20. Jack’s also has some other fertilizer formulations for houseplants and specifically palms, African violets and orchids. Jack’s is very effective and doesn’t have massive salt accumulation like Miracle Gro. You aren’t likely to find Jack’s at Walmart or even your big hardware store, but your local nurseries are likely to carry it.
When you have chosen a fertilizer, just follow the instructions on the package to figure out the dosage. Usually, the feeding rate will be around ½ teaspoon per gallon of water and many fertilizers will even include the measuring tool for you. Once you start fertilizing in the spring, you will want to develop a consistent routine. I generally pick a day of the week and make that my fertilizing day to help me remember to do it. However, there are a few instances in which you will want to skip fertilizing day. If you are using a water-soluble fertilizer, you should avoid an application if the plant’s potting soil has not sufficiently dried out from the last watering. If the plant becomes stressed because of overwatering it will not be able to take in the nutrients, so you are only going to fertilize when the plant is also ready to be watered. If your plant is experiencing any kind of major stress, like a pest infestation, drought or is in a hot environment, you are going to want to forgo feeding the plant until it returns to a normal condition. Adding fertilizer at these times can add to the stress the plant is undergoing and cause it to decline even faster.
Just like some other houseplant cultivation practices, fertilizing becomes a simpler process over time, and while getting your plants these necessary nutrients is important, the execution is not all that complicated. For more information on houseplant care check out some of our other articles Watering Houseplants 101 and Spring Cleaning: A Guide to Repotting Houseplants. Happy fertilizing!
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