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How to Fertilize Houseplants

I’ve gotten so many questions about how to fertilize plants over the past months that I’ve decided to write a post on fertilizing.  How to take care of a plant indoors, and how to fertilize, can be an overwhelming thought.

How do you know what fertilizer to use?  What strength?  How often should you fertilize?  What do the numbers mean on the fertilizer label?

After reading this post, you should feel more empowered with basic fertilizer knowledge and know how and when to properly fertilize your houseplants.

Why Should You Fertilize Houseplants

In nature, plants constantly replenish their nutrients from decaying organic matter.  As leaves, branches, animal life, and other organic matter decompose, they’ll slowly release and recycle their nutrients into the soil and make it available for plant growth.

In your houseplant pots, the soil can be quickly depleted of any nutrients rather quickly and it will be your job to fertilize.  Most houseplant potting mixes are soilless mixes so they intrinsically have little to no nutrient levels, and many have added fertilizer that will support plant growth for a few months.

Fertilizing and soil chemistry is a very complicated topic, but this post will cover very basic knowledge that will be helpful for you and your houseplants.

Basic Fertilizer Information

When you look at a fertilizer label, you will see 3 numbers.  For example, 10-10-10.  These represent the 3 primary nutrients that plants need to grow.  Soil nutrients are classified based off of the amount that they are needed by plants.  The following list shows the most important plant nutrients in decreasing order of usage by plants :

Primary nutrients are needed in the largest quantity and these are:  Nitrogen (N), Phosphorus (P) and Potassium (K).  The 3 numbers on every fertilizer label is called the NPK ratio where N, P, and K represent the chemical symbols for those elements.

Secondary nutrients are needed in smaller amounts, but are still important.  These include Sulfur (S), Calcium (Ca), and Magnesium (Mg).

Micronutrients are only needed in very small amounts, but they are still very important.  Micronutrients include Zinc (Zn), Iron (Fe), Copper (Cu), Manganese (Mn), Boron (B), Molybdenum (Mo), Chlorine (Cl).

In very simplistic terms, here is an explanation of NPK:

Nitrogen (chemical symbol N) is needed for growth of leaves.

Phosphorus (chemical symbol P) is important for flower and root development.

Potassium (chemical symbol K) helps to bolster disease resistance, cold tolerance and protection against drought.

So when you see a fertilizer label, for example with 10-15-10, this is telling you the NPK ratio. This tells you that the fertilizer container 10% Nitrogen, 15% Phosphorus (expressed as phosphate), and 10% Potassium (expressed as potash), respectively.

When to Fertilize Your Houseplants

In general, you should only fertilize your houseplants when they are in active growth.

In my climate, most of my houseplants are in active growth from approximately February through September or October.

During the dismal winter months, with short days and reduced light, I stop fertilizing.  The only exception I make are for moth orchids if they are growing a flower spike.

When NOT to Fertilize Your Houseplants

Fertilizing is not a fix for poor cultural conditions!  If you have a plant in poor light that is not growing, your focus should not be fertilizer.  You should first position your plant in a brighter area.  Light is paramount for growth!  First fix your plant’s lighting conditions, and then when it starts to grow, you can further support the growth by fertilizing.

In the same way, if you have a plant that is not flowering, don’t think that adding a fertilizer high in phosphorus will help you.  Fertilizer will not cause your plant to bloom.  Ensuring the proper light for your plant WILL!  The fertilizer will simply increase the size and amount of your flowers.  When you have the combination of appropriate light along with an appropriate fertilizing, then you are winning!

Do not fertilize your houseplant if it is in a period where it is not growing.  Most of my houseplant growth pretty much comes to a halt in the winter months, so I completely cut out fertilizing when light levels are low.

If your plant’s potting mix has gone completely dry, don’t apply fertilizer, especially if it’s at full strength.  You may burn the plant roots.  A good idea would be to first moisten the soil with plain water first, and then come back and fertilize later.

Fertilize with Every Watering

This is my preferred method to fertilize houseplants.  You could fertilize at full strength at whatever frequency the fertilizer label recommends, but I prefer to fertilize with every watering at a diluted strength.  Why?

Number 1 reason…I don’t have to remember when I last fertilized since I’m doing it every time!

There is a much lower risk of burning your plant.

It is a much more natural way for a plant to receive nutrients.  In nature, plants receive nutrients slowly from decomposing organic matter.

Many fertilizers are designed to be used at every watering.  If you have one of these fertilizers, simply follow the directions!

If you have a fertilizer label that says to dissolve a teaspoon per gallon per month, I would recommend reducing it to one fourth and use that every time you water.  So add 1/4 of the amount.  Or if the fertilizer label says to add a certain amount and use it every 10-14 days, reduce that quantity in half to use for every watering.

Every so often, I would recommend watering your plant with just plain water, even during the growing season.  This will help flush out any excess fertilizer salts that may be building up in your pot.  Terra cotta pots are notorious for fertilizer salt build up since they are so porous.

Also, you may see some crusty buildup on the surface of your soil, or along the inside perimeter of the pot.  This indicates excess fertilizer salts.  Flushing with clear water every so often, once every month or every few weeks, will help with this issue.

Types of Fertilizer for Plants

I have several recommendations of fertilizers in this section and have used all of them with great results.   All of them are available on Amazon.

I’m currently using a Schultz All Purpose houseplant fertilizer that is 10-15-10.  Some fertilizers are more balanced in their NPK ratio and might be 10-10-10 or 20-20-20.  Plants can’t read labels.  Experiment with different fertilizers and see which one works best for you.  I use this all-purpose fertilizer for most of my indoor plants.

There are some specialty fertilizers which you may choose to branch off into once you get comfortable with fertilizing.  Here are some of the ones that I use:

For my moth orchids, I use the Grow More 20-10-20 formulated especially for Phalaenopsis orchids.  If you are interested in being able to grow moth orchids (even if you were a previous orchid killer!), I recently wrote an eBook, Moth Orchid Mastery.  Check out the reviews and what others had to say!

For my air plants (Tillandsia) and bromeliads, I use the Grow More 17-8-22 fertilizer which has produced amazing results for me!

For cacti and succulents, I’m currently using a Schultz 2-7-7 formulation.

My African Violets respond very well to the Optimara 14-12-14 fertilizer made specifically for these plants.  They absolutely thrive on this and the growth and flowering has been out of control!

If you have any edible plants that you are growing indoors, or even outdoors during the warm weather, I like to use only organic products.  For my citrus plants and any vegetables, I really love using the following products:

Neptune’s Harvest Fish & Seaweed Fertilizer 2-3-1 is amazing to use regularly on any vegetable plants and also on citrus.  My lemon, orange and lime potted plants love this stuff!

Morbloom 0-10-10 is a great fertilizer that I use every summer on all of my potted flowering plants outdoors.  It is made from fish emulsion but does not have any odor to it, which is a plus!  Actually, on my citrus plants, I use a combination of this fertilizer and the Neptune’s Harvest fertilizer above.

Another fertilizer I like to use for my citrus plants is Citrus Tone 5-2-6 which is organic as well.  I follow the instructions and apply it three times a year, but then I also use the two fertilizers above as well.  Citrus are very heavy feeders!

One of my favorite fertilizers ever, especially for potted outdoor annuals and plants is Osmocote.  This is a great product because it can be used for indoor plants or outdoor plants, and it is time-release.  The fertilizer itself looks like little balls and you simply mix it into the soil, or add to the surface of the soil and gently mix in.  It will slow-release fertilizer over the course of a few months and you don’t have to remember the last time that you fertilized!  I use this product for all my summer potted plants outdoors, but it would be wonderful for indoor houseplants as well.

Lastly, I feel like I can’t get away with not mentioning Miracle-Gro which is a great product as well.  It is particularly great for foliage plants since it is very high in nitrogen.

Final Thoughts on Fertilizing

Fertilizer is NOT a fix for improper light and poor watering habits.

You should ensure that your houseplants have the appropriate lighting for the variety of plant that you have, and that you are using proper watering techniques FIRST.  Once you have these down, then you can focus on fertilizing your plant in order to achieve the best growth that you can in our artificial indoor conditions!

Fertilizing a plant that is in poor light can cause more harm than good.

If you have a flowering houseplant that is not flowering, the reason is likely not enough light.  Fertilizing your plant will not cause it to bloom.  Moving it to higher light will.  The fertilizer will simply enhance the blooming.

I like to take plastic gallon jugs and premix the fertilizer solution ahead of time.  This way, I have it ready when I need to use it.

Hopefully this post has helped you understand some important aspects of fertilizing your plants.  Comment below with any questions!