Indoor Plants and Fertilizer: What You Need to Know

Indoor plants require several things they can’t get outdoors, and one of those things is fertilizer. Many a would-be gardener has written themselves off as a black thumb simply because they misunderstand how to apply fertilizer, or how often. 

So, what do you need to know about fertilizing indoor plants? Indoor plants must be fertilized periodically throughout the growing season. Many different kinds of fertilizer are available for houseplants, and different species of indoor plants require different levels and types of fertilization. 

The difference between knowing about how to fertilize your indoor plant properly and not knowing is sometimes the difference between a dead houseplant and a live one. Read on to find out more about how to fertilize indoor plants for optimal health and vitality. 

Two Types of Plant Killers

People who try to keep houseplants alive typically fall into two major categories: those who forget their houseplant is a living thing because it doesn’t cry out for attention, and neglect it to death, and those who kill their houseplants with kindness by overwatering or too much fertilizer. 

In both cases, indoor plants can be kept healthy more easily by keeping a record of when and how often you both water and fertilize your houseplants. 

In the case of the neglector, keeping a record will help you monitor how often you’re really tending your plants. 

But in the case of the kind plant killer, keeping a record will help you know how long ago and how often your plant has been fertilized, so that you don’t reapply too soon and start causing damage to the plant. 

Fertilizer is Vital for Healthy Indoor Plants

Fertilizer is vital for the health of indoor plants because, unlike outdoor plants, indoor plants cannot glean nutrients from the soil other than the soil that they are potted in. Eventually, this soil becomes depleted of nutrients if the plant isn’t fertilized. 

Once that happens, the plant slowly starts to starve to death. This can lead to a yellowing of the leaves or other cosmetic flaws, which many novice gardeners mistake for a lack of water. 

Instead of giving the plant the food and fertilizer it needs, the gardener gives extra water instead, which typically only causes more problems, not less. The plant dies, and the gardener mistakenly comes under the impression that they have a black thumb. 

In reality, the gardener does not understand the requirements of the plant they are caring for. Many people buy a plant and think that as long as it has dirt and regular watering, it’ll be fine. The truth is that such plants will do okay for a little while, but when they run out of food, they will fail. 

Elements that Indoor Plants Require

Along with water and light for photosynthesis, indoor plants also require a variety of elements in order to use nutrients. Plants derive oxygen, nitrogen, and hydrogen from the air, but they also need access to the following soil-bound trace nutrients in order to thrive:

• Zinc
• Copper
• Manganese
• Cobalt
• Iron
• Boron
• Molybdenum

The three most important elements plants must have access to in order to grow properly are phosphorus, nitrogen, and potassium. 

The reason that these elements, also known in botany as macronutrients, are so important is that they form the building blocks of the amino acids that eventually form plant cells. Without amino acids, there’d not only be no plant cells, but there’d also be no living organic cells, period. 

The three major macronutrients in gardening are responsible for the following functions in plant growth:

• Nitrogen is used to create a plant’s foliage and keep it healthy.
• Potassium is used by a plant to create buds and blossoms.
• Phosphorus is used to strengthen and expand a plant’s root systems.

What Does Fertilizer Do?

The main purpose of fertilizer is to add nutrients and trace minerals to the soil that the soil might not otherwise have on its own. In outdoor plants, fertilizers are typically used to amend native soils in order to make them more productive.

However, the role of fertilizers in indoor plants is a bit different. Because they have access to static potting soil that is not replenished by nutrients through decomposition or other means (which occurs in native soil) they are limited in their nutrient pool. 

Fertilizers in indoor plants combat the soil in houseplant pots becoming leeched of nutrients by adding nutrients back into the soil that the plant is taking out. In this way, fertilizers help mimic the natural outdoor cycles of soil being revitalized by new organic material being broken down. 

Making sure that the soil has plenty of nutrients not only keeps the indoor plant from starving, it also means the plant has to use less energy trying to feed out of a poor soil; this leaves energy resources to produce flower blooms or fresh foliage, rather than struggling to survive. 

How Often to Fertilize Indoor Plants

How often indoor plants should be fertilized depends heavily on the type of plant it is, and what kind of soil it is used to having. Some plants require much more intensive fertilization than others in order to maintain good growth or to produce blooms. 

Fertilization of houseplants is typically done between every two weeks to once every few months, depending on the species of plant and what kind of fertilizer you’re using. 

For best results, if you have an indoor plant, you should research the specific type of plant that you own to figure out its best individual needs with regards to fertilization. Some houseplants thrive on neglect and can survive with minimal fertilization, others are much more delicate. 

Indoor plants should be fertilized regularly through the growing months but should not be fertilized during the winter. This is because the plant is supposed to be dormant and should not be expending any energy in taking up nutrients or trying to produce growth. 

Types of Fertilizer for Indoor Plants

Fertilizers for indoor plants tend to come in one of three categories:

• Liquid fertilizers
• Slow release fertilizers
• Granular fertilizers

Plants can’t tell the difference between these kinds of fertilizers, as they absorb the nutrients the same either way, but they can offer some pros and cons for houseplants. 

• Liquid fertilizers are applied as a spray and are typically taken up through the plant via its leaves and foliage, where it can use the nutrients right away. 
  
  However, unlike granular fertilizers that are more slowly absorbed into the plant through the roots, liquid fertilizers don’t provide long-term fertilization. Another downside to liquid fertilizers, other than the frequency of application, is that it can be easy to over-fertilize. 

Liquid fertilizers are best used as a way to quickly correct malnutrition in a plant once its discovered, rather than used as a regular fertilization method. 

• Granular fertilizers can offer a more long-term solution to fertilization but come with their own caveats for regular use. 

Granular fertilizers can be considered a “hot” fertilizer, and if you don’t distribute the granules evenly enough, this can leave “hot spots” of rich nitrogen and potassium in the soil, which the plant’s roots will steer away from in self-preservation. 

Granular fertilizers are also not as consistent a fertilizer as liquid fertilizer–liquid fertilizers are consistent in composition, whereas a granular formula will have differing levels of nutrients from granule to granule. 

However, granular fertilizers are easier to store in a stable condition than liquid fertilizers (which have a tendency to separate, especially if stored in the cold), and granular fertilizers are much cheaper than liquid fertilizers. 

Granular fertilizer is also easier to find in bulk and less expensive than liquid fertilizer. 

• Slow release fertilizers are granular fertilizers that have been treated with a coating that causes the granules to break down and absorb into the soil more slowly. The polymer coating resists moisture retention in the granule, which breaks down in accordance with temperature. 

One of the big benefits of slow release fertilizers is environmental. Unlike water soluble fertilizers, which can leech easily into the water table and can be introduced into waterways, slow release fertilizers are released so slowly they are mostly used up by the plant.

Slow release fertilizers are also easier to apply than liquid fertilizers (no mixing required) and store easily. A downside of this fertilizer type is that once it has been applied, it is difficult to adjust without tossing the entire pot of potting soil and starting over fresh.  

Signals of an Unhappy Houseplant

Sometimes you can just look at an indoor plant and be able to tell that it is not doing well, and many ailments of indoor plants can be caused by fertilizing too much or not enough. Symptoms of neglect or malnutrition in a houseplant show up in the following ways:

• Too many leaves, no blooms: If your indoor flowering plant has a bunch of green leaves on it, but no buds or blossoms, this is typically due to having too much nitrogen in the soil, which means you’re probably fertilizing too much
  
  
• Scorched-looking leaves: If the leaves of your indoor plant appear burned, this can be either from too much direct sunlight or from too much fertilization. 

• Brown leaf tips: If the leaf tips of your indoor plant are turning brown, this is another sign of too much fertilization. 
  
  
• Spindly or weak-looking plants: If your indoor plant just looks feeble and not vigorous in comparison to other specimens of its type, and you haven’t been fertilizing on a regular basis, you could be dealing with lack of proper light, lack of food, or both. 

Too Much Fertilizer is Not Good Either

As you can see from the previous section, over-fertilizing a plant can be just as detrimental as under-fertilizing it. Like humans, plants can suffer poisoning if they are exposed to some vitamins and minerals in concentrations that are too high for them to adequately absorbs. 

This symptom most often presents as a brownish discoloration to the leaves and can be prevented by fertilizing only when needed, and on a regular schedule. 

Remember once fertilizer has been added to a potting soil, it is difficult to remove, so if you suspect too much fertilizer is hurting your plants, the best solution may be to repot the plant by removing the soil that is over-fertilized and add 

One of the downsides of liquid fertilizer sprayed on plants is that excess moisture on the plant leaves left by water-based fertilizers can cause mildew or other leaf problems, such as fungal leaf spots or anthracnose. 

Dilution is Key to Avoiding Burned Plants

One solution for accidentally over-fertilizing plants is to dilute the suggested mixture of fertilizer (at least with regards to liquid fertilizers) by half and dilute the other part with pure water. 

It is better to fertilize with a weaker mix on a more frequent basis than it is to overfertilize with 
a mixture that is not diluted enough, which will be drawn up into the plant too quickly for it to utilize properly, and also burn it from the inside out. 

This process is called phytotoxicity and is also referring to whenever plants are inflicted with a chemical burn or chemical-based damage. 

Natural Fertilizers

While you can use pre-mixed fertilizers in any of the major types (liquid, granular, slow release) it can be advantageous to look into natural solutions to fertilize your indoor plants. 

Not only are these options easier on the environment, but they also degrade naturally in the soil of your pot, letting the plant take up nutrients at its own pace rather than being force-fed them. 

These organic compounds are a good food for plants because they most closely replicate a plant’s natural diet in the wild (namely, detritus on the surface of the soil that its. 

Using these natural fertilizers can also help you use up materials from the kitchen that would otherwise go to waste. 

Here are some good options for natural indoor fertilizers:

• Crushed eggshells: Many people use eggs in their kitchen, and crushed eggshells are one of the best ingredients for a homemade fertilizer. Once ground up into a powder, eggshells can be applied to the soil of an indoor plant to increase its uptake of calcium.
  
  Calcium is vital for healthy plants in building strong cell walls, and the sharp edges of crushed eggshells can also be spread at the base of plants in order to deter pests from trying to climb 
  the stem. 
  
  These edges are not sharp to people, but on the microscopic level, they form a sort of razor-wire barrier against insects that would do harm to your plants. 

• Used coffee grounds: To add much-needed nitrogen to the soil of your indoor plants, sprinkle used coffee grounds on the surface of the soil and allow them to leech into the soil each time you water the plant.
  
  It is important that you utilize used coffee grounds, rather than fresh grounds, since fresh coffee is very acidic, while coffee that has been run through a percolator has a neutral pH and is safe for indoor plants. 

• Used tea bags: Like used coffee grounds, used tea bags can be clipped open, and their contents can be strewn onto or gently mixed with the surface soil of a potted plant. This amends the soil with nitrogen, which the indoor plants need to grow.
  
  Mixing used tea and coffee grounds into your indoor plants also gives the plants a slow-release form of food, as these grounds will continue to leech nutrients into the soil as they are organically broken down. 
  
  
• Epsom salts: Supplementing indoor plants with Epsom salts is a great way to encourage them to put out blooms and also improves the vibrant green color of their foliage. This can result in a much more healthy-looking houseplant that performs better.
  
  The magnesium present in Epsom salts helps the plants to better take up other nutrients in the soil, such as phosphorus and nitrogen. 
  
  To apply Epsom salts, dissolve a few teaspoons of the salt in water and then water your indoor plant as usual. This treatment should be repeated once or twice a month as needed. Epsom salts are best used as a spray for foliage but can also be incorporated directly into potting soil when a plant is first being potted. 

Commercial Fertilizers

Along with natural fertilizers, there are hundreds of commercially available fertilizers on the market for indoor plants in all varieties, from liquid fertilizer spray bottles to bulk bags of granular fertilizer. Commercial fertilizers can be found in most home improvement and gardening shops. 

These fertilizers all have different application methods and dosages, so it’s important that once you choose a fertilizer that you read the labeling carefully to make sure that you are dosing the plant at the right level for optimal success. 

It is also important to read fertilizer labels for safety reasons, as some fertilizers (especially non-organic ones) can be poisonous to household pets or children, so make sure that any chemicals you have, fertilizer included, are kept well out of reach. 

One major benefit of commercial fertilizers is that they are labeled, which takes the guesswork out of how much fertilizer you should use and how often. Each commercial fertilizer should come with this information already presented on the label, which makes things much simpler. 

Tools for Applying Fertilizer

Tools for applying fertilizer typically break down into two major categories: sprays and spreaders. 
Sprays are used primarily for liquid fertilizers, while spreaders are used for granular and slow
release fertilizers. 

The reason you want to use a spreader for granular fertilizer, rather than using your hands, is that if you have residue from this fertilizer on your hands and your hands later get wet, it can cause chemical burns or irritation. 

Most commercial fertilizers come in either a container designed for spreading (with a “salt shaker” sort of top) or in a ready-to-use spray bottle.

If you mix up your own homemade liquid fertilizer, you’ll need to acquire an empty spray bottle to keep your mixture in when not in use. Be sure to label this mixture carefully to prevent it from accidentally being used for the wrong purposes. 

Organic vs. Inorganic Fertilizers

When choosing fertilizers for indoor plants, you can choose to either use an organic fertilizer or a non-organic fertilizer. 

Organic fertilizer is basically organic compounds that are used in decomposition to amend the soil, and typically contain both carbon and hydrogen among their components (one of the defining aspects of organic material). 

Inorganic fertilizers can provide a quick boost of nutrients to ailing plants but aren’t conducive to creating a long-term healthy soil environment for your indoor plants. Inorganic fertilizers are susceptible to leaching and can cause environmental damage by introducing too much nitrogen into the soil (or the water table if they are disposed of improperly). 

Organic fertilizers can be a little less effective as temporary measures, but in the long-term are a better option for actually building up the health of your potting soil.  

Fertilization by Season

Indoor plants should be fertilized differently based on what type of the year it is. These plants should be fertilized most regularly and strongly during spring and summer, which are the major growing season for all plants. 

While plants should still be fertilized somewhat during fall, you should stop all fertilization of plants during the winter, as this is a dormant period, and you should not be encouraging plants to put out new growth during this time of the year.

New growth is especially susceptible to sharp temperature swings, diffused weak light, and the other environmental perils that come with a winter season. 

For this reason, you should withdraw fertilizer during this season to encourage the plant to “sleep” until the spring and mimic its natural growth cycle. 

Keeping a Fertilizer Record

The best way to prevent either under-fertilizing or over-fertilizing of your indoor plants is to keep a record of when and how much you have fertilized them. Nobody can remember everything, so why add one more thing that your brain has to remember when you can just write it down? 

This is especially important if you have many houseplants or houseplants with very different care requirements, as it can be difficult keeping track of which ones have been cared for and which ones haven’t. 

In your fertilizer record, record the following for each data entry:

• Species/specimen number (if you have more than one of a species of plant)
• Date fertilized
• Type of fertilizer used

By writing this information down, you’ll be saving yourself the hassle of keeping track of it mentally. 

Know Your Plants for Optimal Fertilization

The most important aspect of fertilizing indoor plants is knowing exactly what kind of plants you have. Plants are as individual as species of animal as far as their environmental tolerances and what they need to really thrive. 

An indoor cactus or a carnivorous plant like a pitcher plant will require very different fertilization than a pothos or bromeliad, so knowing what kind of plant you have is important for giving them proper care. 

Too many people buy an indoor plant as an impulse purchase at a home improvement shop or grocery store without knowing anything about the species of plant they just acquired. You should never throw out the little care marker that comes in the pot you bought your indoor plant in. 

Instead, save these little markers so that you can refer to them later in case you forget exactly what kind of houseplant you’re dealing with. Having this information handy for reference makes the difference between a mediocre houseplant and a well-kept one. 

Mineral Sensitivity (Watch Water Too) 

Indoor plants can be sensitive to the minerals used in fertilizer, but it is also important to know your plants well because some plants do not do well with the chemicals used to process tap water either, such as chloramines and fluoride.

These chemicals, while good at making raw water safe for human consumption, can have detrimental effects on certain houseplants when they take them up. Some plants are pretty hardy against the chemicals in tap water, while others can be completely killed by them.  

A few examples of plants that don’t do well with tap water are carnivorous plants, such as Venus fly traps and pitcher plants, as well as dracaena, which is a popular houseplant. In many cases, plants that are sensitive to tap water will say so on their care sheets/markers. 

If you have a plant that is sensitive to mineral or tap water, you’ll need to water it with distilled water or rainwater in order to keep it healthy and happy. Installing a rain barrel in your home can ensure that you have plenty of clean, chemical-free water to offer your plant along with fertilizer. 

Keeping Indoor Plants Requires Attention to Detail

Many people write themselves off as black thumbs simply because they killed a bunch of houseplants by not taking care of them properly. This is not because indoor plants are difficult to care for; rather, they simply require more care than a “set it and forget it” attitude. 

Because an indoor plant will not cry out for help until it is is very sick, and you can tell by its visible symptoms that it needs help, it’s important for you as its caregiver to be proactive and offer the things it needs to do well before it is sickly and malnourished. 

If you take the opportunity to read up on a type of plant before you purchase it and learn what kind of fertilization and other care it needs, you greatly increase your chances of indoor gardening success.