Best Fertilizer for Indoor Plants and Containers

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Robert Pavlis

There are so many types of fertilizer it’s hard to know which one to use. Which NPK ratio is best? Is one brand better than another? Organic vs synthetic. Soluble vs slow release. This all seems so complicated, but in this post I will simplify the whole process of selecting the best fertilizer.

Best Fertilizer for Indoor Plants and Containers
Best Fertilizer for Indoor Plants and Containers

Fertilizer for Indoor Plants and Containers

Most indoor plants and container plants are grown in a soilless mix that is mostly peat moss, decomposed wood or coir. All of these function about the same way and none of them provide significant nutrients for plants. That is why the gardener needs to add fertilizer, and the best fertilizer for indoor plants and those in containers is the same.

It should be noted that we DO NOT feed plants. What we do is add nutrients to the soil mix and let plants absorb it. We actually feed the soil.

Provided that the soil contains enough of each nutrient, plants will grow fine. If the amounts of nutrients get too high, they will damage roots which shows up as damaged leaves. If the nutrient levels are too low, the plant will just not grow properly.

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There is No Such Thing as Plant Specific Fertilizer

Use google to look for house plant fertilizer. You will find numerous products with differing nutrient ingredients – the NPK value. I found 15-30-15, 18-6-12, 10-10-10, 11-11-18 and 10-15-10. These are all made by experts in house plant fertilizer. How can each one be the ‘best’ for houseplants if they are all different?

They can’t!

The reality is that there is no such thing as the right fertilizer for any plant. Remember that when we fertilize we add nutrients to soil. The plant, for the most part, takes what it needs. It does not really care what ratio you use provided that there is no deficiency.

The other point is that there are many types of house plants. Very few have been studied in such great detail that we know what makes them grow best in every growing condition.

Don’t be conned by marketing. There is no such thing as specific fertilizer for tomatoes, orchids, house plants, African violets, cactus, or any other type of plant.

Proper NPK Ratio

The NPK ratio is the amount of nitrogen, phosphate and potash (potassium) in the fertilizer. Read more about that here: Fertilizer NPK Ratios – What Do They Really Mean.

When plants are analyzed for nutrients, the average amount is in a ratio of about 3-1-2. That means this would be a good average ratio for providing nutrients. So the fertilizer could be a 3-1-2, or 6-2-4, or 9-3-6 etc. Each of these has the same ration of nutrients.

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It does not have to be exactly this ratio, but something close is the best choice.

Myth About Blooms and Root Growth

A piece of common advice says:

  • Nitrogen is used to grow foliage.
  • Phosphorous encourages root growth.
  • Potassium results in more blooms.

This gives the impression that adding more potassium, for example, will produce more blooms, and that is not true unless potassium levels are too low. All of the nutrients are needed to grow leaves and they are all needed to grow roots or flowers. If any one of them is missing, a plant can’t grow, period. If you provide adequate fertilizer, adding more of one of these nutrients will not grow a better plant.

If you are having problems blooming a plant, consider providing less nitrogen. That will slow down vegetative growth and encourage blooming, but this only works if other conditions like temperature, and duration of darkness are also correct.

Micronutrients are Important

Nitrogen, phosphorus and potassium are the most important nutrients, but there are another 20 some minor nutrients that you also have to add to plants. Choose a fertilizer that includes micronutrients.

Types of Fertilizer

Slow release fertilizer, image from Florida Landscape Doctor
Slow release fertilizer, image from Florida Landscape Doctor

You can get fertilizer as granular, water soluble powder, liquid, slow release pellets and spikes.

Spikes are a poor choice in both containers and in the garden since they concentrate fertilizer in one spot.

Granular forms are usually used in the garden where they dissolve when they get wet.

The best option for indoor plants and containers is soluble powder, liquid or slow release pellets. If you are new to gardening I suggest using the former two. They are easy to find in stores, and are easy to use.

Slow release pellets are specially designed to provide a low level of nutrients over several months. They are less work, but it is impossible to know if they have been used up and need to be replaced. The duration of this depends, to some extent, on how much you water. They are easy to use.

Organic vs Synthetic (Inorganic) Fertilizer

There is both organic fertilizer and synthetic fertilizer. But …. there is no such thing as organic nutrients. By the time the nutrients are released by the fertilizer in a form plants can use, it is all inorganic, and plants can’t tell the difference.

The choice between organic and synthetic is a life choice for you – the plants don’t care.

The one problem with organic fertilizer is that some of the material in it has not yet decomposed. Until it does decompose, the plants can’t use it. So you can fertilize today, and the plants can’t get any nutrients. One product  I looked at contained “Alfalfa Meal, Kelp Meal, Neem Seed Meal, Oyster Shell, Azomite, Fish Bone Meal.” None of these can be used by plants until they decompose, and you can’t tell when that happens.

The advantage of synthetic fertilizer is that plants have immediate access. A good quality product also lists the micronutrients in the product, but organic fertilizer rarely provides detailed information about micronutrients.

Does Brand Matter?

I am sure that there are better brands. The problem is that you have no idea which they are. Just because a brand is popular does not mean it is a good quality brand, and endorsement on social media means nothing since most gardeners don’t know how to test the quality of the fertilizer. Statements like, “it works for me”, or “my plants love it” are of no help.

Fertilizer technology is fairly simple and you can expect that most synthetic fertilizer products, from reputable companies, are of similar quality. I usually buy what is on sale.

Match Fertilizer to Your Tap Water

What is the pH of your tap water? If you have either high carbonate levels or a high pH, it is a good idea to use an acidic fertilizer which will modulate the high pH to come extent.

Some fertilizer is labeled as being acidic, but most aren’t. You can look at the ingredients. Fertilizer using ammonium as the nitrogen source tend to be acidic. Sulfate is also acidic.

How Much Should You Use?

This is a difficult question to answer. The goal of fertilizing is to replace the missing nutrients in the soil, but you have no idea which are missing. The amount remaining in the pot or container depends on how much it is watered, how much water flows out of the bottom of the pot, your water chemistry, and how much your plants have used. These are all unknowns and quite variable.

Fertilizing is mostly a guess. Start a regular fertilizing program and watch the plants. If you use too much, they will show burnt leaves. If you don’t fertilize enough you will get small leaves and fewer flowers. If either of these is a problem, adjust your fertilizer amounts.

Stop fertilizing when plants are not healthy because fertilizing sick plants only makes them worse.

Use less fertilizer when plants are dormant. For example, cactus need almost no fertilizer in winter since they stop growing.

As a starting point, use 1/2 of whatever the label says. This will work in most cases. Run water through the pot once a month to remove excess fertilizer buildup.

Watering Indoor Plants

Here is a bonus video you might like.

YouTube video

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Robert Pavlis

I have been gardening my whole life and have a science background. Besides writing and speaking about gardening, I own and operate a 6 acre private garden called Aspen Grove Gardens which now has over 3,000 perennials, grasses, shrubs and trees. Yes--I am a plantaholic!

14 thoughts on “Best Fertilizer for Indoor Plants and Containers”

  1. Hello Robert, thank you for your amazing work, both here and on your YouTube channel! I have learned so much from your expert advice.
    I have a question if I may: I want to exclusively use an organic liquid fertilizer as I want to have more control over feeding of my plants (I grow chilies indoors in pots). But what if I used a growing medium devoid of any microbial life, such as Coco peat and nothing else? How would the nutrients from the organic liquid fertilizer become available to the plant in such case? Would they be able to utilize the nutrients? Synthetics would feed the plant directly, do the liquid organics do the same or do I have to incorporate something like worm castings in my mix?
    I’d greatly appreciate your reply. Thanks to you I learned that many things I was told were not true and in fact affected my growing success in a negative way.
    Best regards,
    Daniel

    Reply
    • 1) There is no such thing as “growing medium devoid of any microbial life” – the pots, the plants and your hands are covered in microbes.
      2) Organic liquid fertilizer actually makes it harder to “control feeding”. You never know how much of the nutrients in the organic matter are available to plants. It is also the most expensive way to fertilize plants.
      3) Organic fertilizer has some plant available nutrients which plants can use right away, the rest needs to be decomposed to release the nutrients. You have no idea how quickly that happens. Any organic material including worm castings works the same way.
      https://youtu.be/lo7Smm2-mDI

      Reply
  2. The article has a valuable content, which has helped me a lot in understanding Best Fertilizer for Indoor Plants and Containers. I think if anyone wants to become expert in Best Fertilizer for Indoor Plants and Containers, then he needs to read your content consistently.

    Reply
  3. Hi Robert,
    I have taken a concerted effort to invest my time, not just my money which I have wasted over the years in houseplants, to understand how to care for my houseplants. I found your article super informative and the biggest take away is that I will stop driving myself crazy trying to figure out which fertilizer works best for which houseplant. I have been doing research for days and days on end, and I feel like I’m no further ahead in understanding what works. I’ve just been going around in circles. But I do love my houseplants and I’m tired wasting money because I’m buying plants every year only to have some last one season and that’s it. Mind you, I buy them cheap but it all adds up. Just last night I counted six (6) different fertilizers with different NPK ratios, earnestly trying to provide the ‘best’ fertilizer for each of my more expensive houseplants. I will no longer be doing this, thanks to your article. I will use what I already have and will watch/wait to see how my plants react. I feel like you are giving sound advice I and wish I had come across your article sooner and save myself a few dollars. There’s still a good two months left before the cold weather sets in. That’s still plenty of growing time for my houseplants. Thank you for sharing

    Reply
  4. Just a question. Using a synthetic fertilizer you say that, the plant does not know the difference. As I have read the synthetic fertilizers were created from petroleum by-products, would you recomend still to feed these fertilizers to a plant I would later harvest and eat?

    Reply
    • They can’t be made from petroleum products. The nitrogen products are made from air, and P and K are mined minerals.

      Yes use them – they are identical to what you get from compost.

      Reply
  5. Excellent article as usual. Where does iron fit in to the fertilizer for outdoor shrubs? Thank you for all the help you provide in understanding this wonderful hobby of gardening. Stay healthy,,,,Jack

    Reply
    • Iron is usually not deficient, but in alkaline soil plants may have a problem getting enough. Adding more in this case not really work since it just gets tied up in soil. Adding organic matter ads chelates that make iron more accessible.

      If iron is deficient, a soil test can shiw this.

      Reply

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