Fertilizing House Plants

Fertilizing your house plants gives them the essential nutrients they need for promoting and maintaining new and healthy growth. Knowing when and how to feed your plants can be quite challenging. Not only do you need to worry about when to feed your plants, you also need to make sure you’re using the correct amounts to avoid over-fertilising and burning your precious plants.

The Importance of Fertilizing House Plants

We need to fertilize our plants in order for them to look and grow their best. As our house plants grow, they extract the essential nutrients they need from the soil. Since our plants are inside, they’re not exposed to external nutrients like they would be in their natural environments. Fertilizing your house plants will replenish the soil with the essential nutrients that have been lost during the growing months.

How Often Should you Fertilize Indoor Plants?

Different house plants require different needs and when it comes to fertilizing amounts and when to fertilize, it can become tricky. But in general, most house plants require a similar amount of fertilizer. With most house plants, you should be fertilizing them monthly during the growing period (Spring-Summer). During the cooler months, fertilizing isn’t required. This is because our house plants aren’t actively growing, therefore, they aren’t using up the nutrients in the soil and you risk over-fertilizing your plants.

You should also consider the last time you repotted your plants. If you potted up some of your house plants into fresh soil at the start of spring, they won’t require any fertilizing for about 2 months. This is because most potting soils already contain fertilizer in them.

Types of Fertilizer

There are a few different types of fertilizers that you can get for your indoor plants. They range from liquid fertilizers to granules, sticks, organic and slow-release varieties. The best fertilizer that you should use for your indoor plants are the liquid, organic and slow-release varieties. Even though the stick and granule fertilizers are convenient and easy to use, the nutrients don’t distribute through the soil as well. Once you insert them into the soil, you don’t have any control over the release.

Liquid Fertilizer

Liquid fertilizers need to be diluted with water and applied by a watering can. Some fertilizers are meant to be used every time you water or every other time, this depends on the label instructions and the type of plants you have. It’s best to research the type of plants you have to determine how much fertilizer is needed and how frequently. Liquid fertilizers provide a balanced supply of nutrients for a wide variety of plants.

Slow-Release Fertilizer

Slow-release fertilizers slowly release a steady amount of nutrients into the soil over a period of time. The pellets are coated with a plastic resin or sulfur based polymers that breakdown when they come in contact with water. This type of fertilizer can last for up to 6 months from one application and it also helps to eliminate the risk of fertilizer burn. Even though slow-release fertilizer is more expensive to buy, the less frequent applications and how long it lasts, makes the costs even out.

Granular Fertilizer

This type of fertilizer is used more in outdoor gardens. Granular fertilizer is dry pellets of pure fertilizer that you mix into the soil. When you water your plant, the pellets release all the nutrients at once, making it hard to control how much fertilizer your plants get. Although this type of fertilizer is cheaper, it’s best to not use on your indoor plants.

Organic Fertilizer

Organic fertilizer is a fertilizer that is made naturally by or from plants and animals. The type of fertilizer you can get is worm castings, compost, blood and bone, fish emulsion and kelp. Since organic fertilizer is produced naturally, most of the nutrients are readily absorbed by the plants.

What is in House Plant Fertilizer?

House plant fertilizers contain a mixture of macro- and micro-nutrients. The three essential macro-nutrients are Nitrogen, Phosphorous and Potassium, and these are found as ratios on the fertilizer bottle or container. This ratio is called the N-P-K ratio. The N-P-K ratio shows the percentages of each of the nutrients in the bottle. The macro-nutrients in fertilizers for other plants (such as vegetables or lawns) are different to those in a fertilizer made for house plants because they all require different nutritional needs, meaning it’s important to use a fertilizer that is made for house plants. House plant fertilizers should also contain the essential micro-nutrients like zinc, boron and iron.

Best Type of Fertilizer for House Plants

When choosing a house plant fertilizer, you need to put into consideration the kind of house plants you have and their fertilizing needs. Flowering house plants like African violets and Begonias need a fertilizer that is slightly higher in phosphorus (for example 1-3-1 NPK). Regular, non flowering house plants need a fertilizer that is slightly higher in nitrogen (for example 5-5-5 NPK). Some people like to use a seperate fertilizer for their flowering plants and another for their non flowering plants, however, you can get away with using an all purpose house plant fertilizer. Most common all purpose house plant fertilizers are a 2-2-2 NPK ratio.

Some fertilizers will also contain secondary macro-nutrients like calcium and magnesium, as well as micro-nutrients like iron, zinc and boron. These nutrients are only in small amounts but are still essential to your plants health. When choosing a house plant fertilizer, you should check to ensure these nutrients are also included.

How to Fertilize House Plants

When it comes to fertilizing your house plants, it will depend on the type of fertilizer you’re using. Generally, you should follow the instructions on the back of the bottle. However, many house plant will do well when you dilute the fertilizer to half the recommended strength. This will also help eliminate over-fertilizing.

How to Fertilize House Plants with Liquid Fertilizer

  1. Grab a watering can and dilute the recommended fertilizer strength to half strength
  2. Apply fertilizer to the soil and avoid getting any on the foliage. Getting fertilizer on the foliage can cause burns and spots. Using a watering can with a thin, long spout will be able to get closer to the soil and will help avoid the fertilizer getting on any of the leaves.

How to Fertilize using Slow-release Fertilizer

  1. Following the package instructions, sprinkle the fertilizer around the pot on the soil.
  2. Lightly mix the granules into the soil.
  3. Water plant thoroughly to initiate the nutrient release.

For slow release plant fertiliser spikes you should:

  1. Check the instructions on the packaging for how may spikes are recommended for the pot size.
  2. Insert the spikes into the potting mix half way between the plants stem and the pot edge. Push the spikes all the way into the soil just below the surface.
  3. Water plant thoroughly to initiate the nutrient release.

How to Fertilize using Granular Fertilizer

  1. Insert the granular fertilizer about 3 inches into the top layer of the soil.
  2. Thoroughly water the plant to release the nutrients.

Mistakes to Avoid when Fertilizing House Plants

There are two common mistakes people make when it comes to fertilizing house plants and these are over-fertilizing and under-fertilizing.

Over-fertilizing House Plants

Over-fertilizing is a common mistake, especially for beginner plant parents. We love our plants and want them to grow and be their best but our plants can only take in so many nutrients. Plants make their own nutrients, light being the main one.

If you’re using an organic fertilizer, it is less likely to over-fertilize than if you were using a synthetic fertilizer. Synthetic fertilizers can easily cause toxic imbalances and leave behind heavy chemical residue. Not only are synthetic fertilizers concentrated, they don’t contribute to the living environment of symbiotic organisms that help plants thrive.

Common ways to Over-Fertilize

  • Over use of slow-release fertilizers
  • Using a combination of different forms of fertilizers
  • Fertilizer residue left in the soil due to poor soil drainage
  • Fertilizing when soil is dry
  • Continuing fertilizing during dormancy

Symptoms of Over-Fertilizing

  • Lower leaves yellowing and/or wilting. These are also symptoms of over-watering. You should rule out this before flushing the soil because flushing soggy soil can cause more damage to your plants health.
  • Long term salt build up. Salt will slow the flow of water to the plants roots causing the plant to weaken. You will be able to see a crust of fertilizer on the soils surface or it shows as stains on the outside of a terra cotta pot.
  • Too much nitrogen. This will cause leggy growth and can attract unwanted pests. Other symptoms can be leaf loss, slow growth and root rot becoming more common.

Solutions for Over-Fertilizing

  • Feed your plants in moderation. Diluting your fertilizer and stick to a schedule that works for you and your plants.
  • Try and avoid feeding plants that have slow-release fertilizer in the soil.
  • Think about using a fertilizer that has a low NPK ratio.
  • Ensure water flows freely through the soil. Well draining soil helps excess fertilizer leach from the soil. Well draining soil also helps to stop salt build up.
  • Flush the soil. Doing this approximately every 6 months will assist in getting rid of any excess fertilizer from the soil. You may need to do this more regularly if you fertilize heavy.
  • If you spot yellowing leaves or any other signs of over-fertilizing, flush the plants soil.
  • If your plant has been over-fertilized by a slow-release fertilizer or a combination of fertilizers where a slow-release has been used, you should repot the plant into fresh soil. It is difficult to flush the excess salt from active slow-release fertilizer.

How to Flush Soil

  • Water the top of the soil so that it drains freely from the drainage holes. Repeat a few times allowing the water to drain completely in between.
  • For the more delicate plants like Calathea, use conditioned or filtered water.


Under-fertilizing house plants is definitely one that isn’t as common. The symptoms of under-fertilizing are quite similar to those of over-watering, over-fertilizing and other common maintenance issues. Before assuming your plant is lacking nutrients because of slow growth or lack of vigor, you should rule out other possible plant health problems first.

Symptoms of Under-Fertilizing

House plants that are lacking nutrition will often have pale leaves and weak stems. Since all the plants energy is redirected to the newer growth, the lower, older leaves will appear weak or yellow. Plants may also have slow or stunted growth.

  • New growth being a pale lighting green and yellowing lower leaves: This is a sign that the plant is lacking nitrogen and is sacrificing the new growth for production. A nitrogen deficiency can cause plant sickness, slow growth and sometimes leaf loss.
  • Chlorosis, light green foliage and dark green veins: This is a sign that the plant is lacking potassium. The foliage may also have dark spots appear and the leaves become weak and easy to break off.
  • Older growth turning purple and newer growth staying dark green: This is a sign that the plant has a phosphorus deficiency. Lack of phosphorus can also cause the plant to have late or poor flowers, brown leaves, drinking leaves and just general poor health.
  • Yellow and brown spots on the foliage: This can be a sign the the plant doesn’t have enough calcium. This can also cause the plants growth to slow.
  • Weak stems, loss of colour in older leaves, yellow and brown spots with veins staying green: This can be a sign of a magnesium deficiency.
  • Lighter, pale foliage: This can be a sign of a lack of sulfur. This deficiency is quite rare but if it does happen the plant will look a lighter colour with pale green foliage. It may also develop symptoms that are similar to a nitrogen deficiency.

Solutions for Under-Fertilizing

If you think your plant is suffering from a particular nutrient deficiency, you can get a fertilizer that is higher in the nutrient you’re trying to boost. However, it’s best that you use a fertilizer that also contains the other nutrients as well. You don’t want to overload your plant with that one specific nutrient, you should be aiming to use a balanced fertilizer.

If your plant has started to flower and you notice that it’s struggling, it may benefit from a little extra phosphorus and potassium. Any type of Bloom Booster will work for this but you should make sure it includes the secondary macro-nutrients which are calcium, magnesium and sulfur.

If all other possibilities have been ruled out, you can get a soil tester. Soil testing will give you all the answers to any missing nutrients. You want to ensure you don’t fertilize your plants if they don’t need it as over-fertilizing can do more harm than good.