Now that we have covered lighting and watering needs for our beloved houseplants, in our Indoor Lighting Guide and Plant Watering Explained guides, next up is fertilizing. If you don't have access to a backyard or outdoor area, houseplants can add that touch of greenery to your life. While they make it a little clearer when they need more or less light, water, or humidity, indoor plants don't necessarily make it as obvious when they need feeding, or you may have never been taught that certain signs are actually a lack of a specific nutrient. Unlike humans, who can effectively communicate signs of hunger (aka rumbling tummies, hanger - bad-tempered and irritable as a result of hunger, crying, or low energy), plants are not able to express their needs as clearly.
As long as a plant is in the right conditions otherwise, including correct lighting, a proper watering routine, and accurate soil makeup, it can certainly be fertilized.
All plants, both indoors and out, need fertilizer, but the importance of feeding our indoor plants is often overlooked even though it is essential to promote healthy growth. Their needs are entirely in our hands, unlike those plants that live outdoors where nature provides rain and plants can send new roots searching for nutrients. There is a constant stream of nutrients coming in from the soil through these roots at all times. But the soil in containers becomes depleted of nutrients much faster than in-ground soil, due to more frequent watering, which washes out nutrients faster and increases demand for them.
With all that being said, today we are going to go over everything you need to know about fertilizing your houseplants, including why we do it, what it is, what type of fertilizer to use, and finally, how and when to apply it!
For the sake of this guide, although we do mention a little bit about outdoor plants and how fertilizers affect plants in general, please note that we are mainly speaking about fertilizing houseplants.
What is Fertilizer?
Essentially, fertilizer is plant food. It contains the basic macronutrients plants need to grow, which are Nitrogen, Phosphorous, and Potassium, aka NPK. If you haven't noticed yet, there are always three numbers on a fertilizer label, each number corresponding to the appropriate nutrient and always in the order of N-P-K. Along with these primary nutrients, fertilizers also often contain other nutrients that are less critical but still beneficial, such as Calcium, Magnesium, Iron, more micronutrients, and even a percentage of organic matter and fillers. The main nutrients perform the following functions in stimulating plant growth:
Not every fertilizer will have all three macronutrients present as it all depends on what you want your plant to focus on!
Remember how there are always three numbers present on a fertilizer label? These numbers refer to the percentage, by weight, of the three primary macronutrients present in the fertilizer product (e.g. 5-10-5, 20-20-20, 29-0-5, etc.). They can also be read as a ratio, for example, a fertilizer with 5-10-5 means it has 5% Nitrogen, 10% Phosphorous, and 5% Potassium, but it can also be read as a ratio of 1-2-1 of all the nutrients. To help break it down even further, 5 pounds of a 10-20-10 fertilizer would give the same nutrient value as 10 pounds of a 5-10-5 fertilizer.
Fertilizer Tip: In general, if you want to replenish some nutrients in the soil and don't wish to make it too complicated, a balanced fertilizer of 5-5-5 or 10-10-10 is always a safe option. This will provide for your plant's basic needs without too much of any one nutrient.
Fertilizers that contain all three nutrients are known as complete fertilizers (e.g. 5-10-5 or 20-20-20), but those with the same number for all the elements are called balanced fertilizers (e.g. 10-10-10). There are also incomplete fertilizers, which are lacking a number (e.g. 29-0-5), but they are not inferior to complete fertilizers, it simply depends on what your plant needs, or what you wish to encourage more of! If you wish to have an even more tailored fertilizer, there are other specialty fertilizers for certain plants, such as African Violets, Orchids, or Citrus plants, that contain optimized proportions of nutrients for those particular species.
If your soil already has an excess of one of the three nutrients, you could actually be harming your plants by adding more fertilizer to the soil. In this case, an incomplete fertilizer may be the right choice for you.
Just like in most areas of life, moderation is key when it comes to fertilizing plants since it is possible to give your plants too much. Maintaining proper soil pH, which is the soil's degree of acidity or alkalinity, is vital for healthy plant growth. When pH is too low or too high in the soil, nutrients can’t release to the plants, so you may be fertilizing but the plants aren’t getting anything to eat. Improper soil pH can also present itself as Chlorosis (explained more later on), problems with soil microorganisms (all those little guys that help create a self-sustaining environment), tip burn, insufficient Phosphorous (causing issues with photosynthesis), and more. Soil pH is different than fertilizer, but it is definitely something to keep in mind when you are considering the health of your soil!
One final thing to keep in mind: fertilizers are not the same as soil amendments. Amendments alter soil pH and physical characteristics, such as texture and porosity, which will affect plant growth indirectly. These include mediums such as coconut coir, perlite, orchid bark, vermiculite, and more. Fertilizers, on the other hand, change the chemical nutrient makeup of the soil, influencing plant growth directly.
Why is it Important to Fertilize?
In essence, plants need nutrients, especially those that are grown in pots. Nutrients available to a potted plant are strictly those that have existed within the small amount of soil that the plant was potted into, unlike the expanse of soil that outdoor plants have access to. Once the soil becomes depleted of nutrients, as we know occurs faster for container-grown plants due to frequent waterings as compared to those grown in the ground, the plant will suffer. After the nutrients are all used up, the soil will become stagnant (since it is never being replenished), which is why it is important to keep feeding our plants what they need to support their overall health!
All plants will need some amount of Nitrogen, Phosphorous, and Potassium to support their growth, from houseplants to garden plants, especially vegetable plants. You won't be harvesting bushels of tomatoes unless they are fertilized accordingly!
Nowadays, potted plants will usually have some sort of fertilizer or enhancements pre-mixed in with their soil, such as a slow-release fertilizer (i.e. those colourful orbs that often can be seen). Knowing that, in the first days and weeks of owning a plant, they won't need as much fertilizer, generally being ok for up to two months. After this timeframe, they will have consumed the majority of the nutrients and will need to be refreshed to encourage healthy, new growth, which is where you and your fertilizing methods come in.
Which Fertilizer Should You Use?
Not all fertilizers are equal, there is such a thing as using the wrong type for a certain pant, using too much at one time, using too much too frequently, or not using enough. The main thing to look for is that it is indeed a houseplant fertilizer! Next, you can decide which type of fertilizer you prefer, or which your plant prefers, as there are many types of fertilizers out there. Some of these include liquid fertilizer, slow-release pellets, granule fertilizer, fertilizer sticks, fertilizer tablets, and foliar spray.
Although most houseplants can be given an all-purpose, balanced fertilizer, different plants have different needs and some could benefit from a more tailored fertilizer regime. Let's go over the most common fertilizers, including some pros and cons for each:
Fertilizer Tip: If you are using synthetic fertilizer, it would be ideal to supplement with some sort of organic matter every once in a while, such as compost or manure, to maintain soil health!
To determine which fertilizer would be nutritionally best for your plant, we recommend researching the specific needs of your plants. Do you want your plant to mainly focus on leaf production? Are you looking for your plant to produce beautiful blooms? Is the overall health of your plant the most important thing to you? The choice ultimately depends on how particular you prefer to be as well as the type of plant being grown and the soil it is grown in. Take a look at the numbers on the label and compare them to what we described above, as to what each number means! Fertilizer labels often outline what it would be best-suited for.
Typically, liquid and slow-release fertilizers are the best options for indoor plants, fertilizer sticks and granules seem convenient but they won't distribute nutrients as evenly and, once you have inserted a stick into your pot, you have no control over its release. Granular is best for outdoor use!
When & How to Fertilize
Finally, now that we know why we fertilize our plants, what fertilizer is, and which fertilizer options exist, we can now focus on when, as well as how, to fertilize. It truly all depends on any individual plant's needs, as different plants need more of one nutrient than another. However, since most houseplants are primarily grown for their foliage, they can be put on the same feeding cycle and can be treated in a singular way. With that being said, if you wish to look up any specific plant needs, that is also a great way to go! We just want to acknowledge that the process doesn't have to be overly complicated if you don't want it to be.
Despite the fact that plants never actually tell us when they want to be fertilized, as a general rule of thumb, fertilize throughout the growing season, when the days are long and bright, and plants are actively sending out new growth. For the Northern Hemisphere, growing season is typically April through September, or spring and summer, and then most plants are dormant for the rest of the year, meaning we don't want to fertilize them since this can cause fertilizer buildup if the plant doesn't need to utilize additional nutrients, which can result in foliage and root burn. We also want to give our plants a chance to rest and they generally won't need as much water either.
The main thing to watch out for when fertilizing is how often you fertilize each plant. Some plants, such as Succulents and Cacti, really aren't heavy feeders and don't need to be fertilized as often as something like a Peace Lily or Ficus.
Begin fertilizing your plants about 8 weeks before the last expected frost (around mid-March), starting with a more diluted fertilizer mixture than called for for the first few applications, allowing the plant to gear up for the active growth season ahead. Then, although it does depend on the fertilizer and plant you are fertilizing, you can usually expect to fertilize every 2 - 3 weeks from the end of March until around September. Finally, about 8 weeks before the first expected frost of the season, start to taper off and go back to fertilizing at half strength, lowering the frequency of fertilizing as well. This method of fertilizing is for those fertilizers that will be applied at watering, so slow-release and granular would not be used exactly as described here.
Some people fertilize with every watering, but this is often because fertilizers are more diluted, or because the fertilizer itself is of a low dosage. Feel free to play around with your plants and fertilizers but remember, it is possible to give our plants too much of a good thing!
Next, let's talk about how to fertilize your plants, because there are a few key things to take into consideration. So, before fertilizing, go over the checklist below and then consider whether you should fertilize or not:
☑️ Was the plant recently purchased?
- You can usually wait up to 6 weeks before fertilizing a new plant because they were most likely fertilized well in-store.
☑️ Is it growing season or are your plants actively growing?
- Although we definitely encourage fertilizing your plants in general, we want to make sure it is being done at the correct time. Check out the section above referring to when, and how to fertilize!
☑️ Is the plant already stressed?
- A lot of people think that a plant that is struggling just needs to be fertilized, but that is far from the truth. When a plant is stressed, its other needs should be analyzed first - Is it receiving enough light? Is it receiving too much light? Has it been watered properly? Did the soil compact? Is there already a build-up of salts from past fertilizing? Generally, a stressed plant should never be fertilized.
☑️ Was the plant just repotted?
- Wait a few weeks before fertilizing since the new soil most likely has some nutrients in it and we want to avoid the fertilizer harming any delicate new roots that may be forming. Feel free to read more about this in our Repotting Guide.
☑️ Is the soil very dry?
- It is recommended to only fertilize plants that have moist soil, which will keep the fertilizer from burning the roots that are in dry soil as well as helping the nutrients absorb better.
☑️ Has the fertilizer been diluted enough?
- Never pour liquid fertilizer right into a plant directly, it must be diluted at least how the label says to do so, if not more. A build-up of fertilizer salts will definitely do more harm than good, so less is more!
If it is indeed time to fertilize, dilute your fertilizer as needed, or prepare the slow-release pellets or organic material to spread on your plants and apply it evenly to the surface of the soil. This will ensure that it reaches all parts of the plant, rather than just one entry point, therefore giving all of the roots access to nutrients. If it is a fertilizer stake, simply insert as many as necessary and let them do their thing. However, after applying the fertilizer, if it was a liquid, if there is any excess water that drains out, make sure this is dumped or removed, which should happen regardless, because, as we know, we don't want a buildup of fertilizers. Otherwise, applying fertilizer is fairly self-explanatory!
Common Fertilizing Issues
Despite the fact that fertilizing can be straightforward, as long as we know what nutrients our plants need, which type of fertilizer works best, and how to apply the fertilizer, there can be a few hiccups in the process if the steps aren't followed correctly. To close off this fertilizing guide, let's go over the most common issues you may encounter throughout the process:
Just like it is possible to provide plants with too much light, too much water, or too much humidity, it is also possible to overfeed our plants. Although feeding is part of the journey of keeping our plants happy and healthy, giving them too many nutrients can cause fertilizer burn, which is when there is a buildup of fertilizer salts in the soil, resulting in burned leaf tips/edges. If the burn isn't too excessive, you should be able to flush the salts out of the soil by watering fully multiple times with regular water and then holding off on fertilizing for a little while. The plant may or may not fully recover, it all depends on how much damage was done to the roots, but it is best to only fertilize as the product label recommends going forward.
2. Nutrient Depletion Issues
The most common forms of this are Chlorosis and Magnesium Deficiency:
If there is too much of any one nutrient or not enough of a specific nutrient, it can result in issues such as those above, or something like lopsided growth (when there is too much or too little Phosphorous). This is why it is key to figure out what fertilizer will work best for your plant, as well as what you would like your plant to focus on specifically.
3. Fertilizing Stressed Plants
Fertilizing plants that are already stressed, whether that is because of lighting issues, watering issues, humidity issues, or anything else impacting them, can be quite damaging. It is best to only fertilize plants when they are healthy and growing, not when they are struggling. We know that this will most likely only cause further stress to the plant as it won't be able to absorb the nutrients being sent its way, which usually leads to fertilizer burn. Another thing to keep in mind is that plants that are kept in lower light probably don't need as much fertilizer as those grown in brighter light since they will be growing at a slower rate, and their soil will be drying out at a slower rate as well.
All-in-all, fertilizing our houseplants doesn't have to be an overly complex process, as you have read above it is ok to have lots of your plants on the same schedule and fertilizer, it is just important to apply it properly. Despite the fact that this isn't an exhaustive article about fertilizers, this is definitely a great start to your fertilizing journey. Your plants will thank you in the long run by producing lots of healthy new growth!
With that being said, should you have any questions, or need any further assistance, we are always happy to help, feel free to reach out to us at firstname.lastname@example.org or visit us in-store.