Bloom Booster – Fertilizer Nonsense #5

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Robert Pavlis

Bloom Boosters are said to increase the number of flowers on your plant. Nonsense. They don’t work, and can actually make your soil toxic, making it more difficult for your plants to grow.

Bloom booster fertilizer - who needs it
Bloom booster fertilizer – who needs it? Clematis recta growing just fine at Aspen Grove Gardens without any fertilizer. Photo by Robert Pavlis

Bloom Booster – What is it?

Bloom Booster fertilizer is a fertilizer with a high middle fertilizer number – it is high in phosphorus. It may also have a higher than normal amount of potassium.

There is no such thing as a Bloom Booster fertilizer!

Any fertilizer with a high level of phosphorus can be considered to be a bloom booster fertilizer, even if the label just calls it fertilizer.

If you look at the fertilizer numbers in bloom booster products you quickly realize that every manufacturer’s formulation is different. That’s odd! If there was a special formulation that boosts flowers you would think all manufacturers would use the same formula? They don’t, because there is no such thing. These are just packages of fertilizer with the words ‘bloom booster’ slapped on the label.

Soil Science for Gardeners book by Robert Pavlis

Bloom booster fertilizer is nothing more than a marketing gimmick to get you to buy more fertilizer. Many people grow plants for the flowers, and so manufacturers are using your desire for more flowers to sell more products.

Bloom Boosters, Do They Work?

Will a high level of phosphorus increase the number of flowers?

The answer to this question is a bit more complex than it seems. Plants need a certain amount of phosphorus to grow properly. If they get enough phosphorus along with the other needed nutrients, plants will grow to their best ability and produce lots of flowers. If they can’t get enough phosphorus and other nutrients then they under perform.

Adding phosphorus to soil that already has enough phosphorus, or adding it to one that is deficient in one of the other nutrients, will NOT make plants grow better. You are just wasting a natural resource or worse, making your soil toxic.

Adding a bloom booster to soil that already has lots of phosphorus, will NOT increase the number of blooms.

It turns out that in North America, few garden soils have a deficiency of phosphorus, which means that in most of these soils bloom booster does nothing to boost blooms.

If you check bloom booster fertilizers you will notice that some include minor nutrients, along with some nitrogen and potassium. If the fertilizer fixes any deficiency in these other nutrients, plants will perform better. Except for nitrogen, most garden soils do not have a deficiency in these other nutrients.

Do Plants Use a Lot of Phosphorus?

Lets have a closer look at the nutrients plants need. How much phosphorus do plants need?

When rose tissue is analyzed, the fertilizer numbers are in the ratio of 6-1-4 (ref 1). They contain much less phosphorus than nitrogen or potassium. According to Jeff Gillman (ref 3), “there is usually more calcium, magnesium and sulfur in plant tissue than phosphorus”.

So why would a fertilizer ratio of 1-5-1 (Peters Professional Super Bloom Booster), be good for plants?

Phosphorus is critical for plant growth, but plants don’t need a lot of it. The American Rose Society says “Commercial  growers of roses for cut-flower production typically use fertilizers with a 3-1-2 NPK ratio.” I think these guys know how to produce blooms!

Plants do not need high levels of phosphorus to bloom well.

Phosphorus Can be Toxic

High levels of phosphorus in the soil ties up iron so that plants can’t absorb it. This leads to an iron deficiency in the plant, leading to interveinal chlorosis.

Excess phosphorus also inhibits the development of mycorrihizal fungi which are very important to plants. These fungi provide water and phosphate to the plant. For more details on this see Mycorrhizae Fungi Inoculant Products

Excess phosphorus is toxic to plants.

Does Potassium Boost Blooms?

Some of the bloom boosters also provide an increased level of potassium, and some people feel that potassium is important for flower production.

Potassium is essential for plants to grow and flower properly, but high potassium levels will not increase flowering. As reference 2 points out – flowering is controlled by hormone levels – not nutrient levels.

What About Containers?

In reference 3, Jeff Gillman quotes Timothy Broschat and Kimberly Klock-Moore from the University of Florida “Most container grown plants require only minimal amounts of P for optimal growth and … applications of high P fertilizer will not promote either roots or shoot growth in plants as popularly believed.

Don’t use bloom boosters for containers.

Phosphorus Does NOT Promote Flowering

There is little evidence that phosphorus promotes flowering. Bloom boosters are not needed, nor do they do help your plants, unless you have a phosphorus or potassium deficiency in the soil. The only way to know this, is with a soil test.

Stop adding phosphorus to your soil!


1) The American Rose Society – Phosphorus Fallacies – too Much of A Good Thing:

2) The Story of Flowers – The Potassium Myth:

3) “The Truth About Garden Remedies”, book by Jeff Gillman

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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!

37 thoughts on “Bloom Booster – Fertilizer Nonsense #5”

  1. I think you need to do more research amd experiment with atami Blossom builder liquid. Not only does it add size and weight to your crop; it also promotes thicker skin on the fruits and vegetables. I have used it for years and it is remarkable. Not only does it add size and weight; it also makes your crops more fragrant. Im assuming you have never used it. If you had you wouldn’t have wrote this article.

  2. Thanks for the info!
    Where do you recommend getting a proper soil test done in Ontario? I tried one of those cheap kits but I believe I read one of your posts saying they’re useless (I totally agree), but we also don’t have the State Extension offices to send samples to like in the US. Last time I looked, it seemed most if not all services were for farmers with full-fledged agricultural businesses.. I’d love to know where the regular home gardener could get it checked out. (I’m in Niagara if it makes any difference)

  3. I hope you still read this blog. Thanks so much for this. I have a black thumb and killed some succulents, but I “inherited” roses bushes, gardenia, plumeria, and star jasmine when I signed the lease on my house in SoCal—just moved from Colorado. Because I want to be the perfect tenant, I research like crazy. Out of that list, my roses and plumeria are kicking ass, and all old ladies compliment them. (In-ground and thriving in crappy Californian soil.) I love roses and wish I had them everywhere—I’ve studied those the most. But my jasmlne and gardenia—forget it. I had one good year with both, though the jasmine didn’t bloom much. Now? Leaves are falling off of the jasmine (so much for easy care) and the gardenia has chlorosis. Naturally, I tried BLOOM BOOSTER and discovered it has no iron anyway. Lol. That jasmine actually did bloom more than ever, but the leaves are still so-so. Both are container plants.

    I don’t want to solicit a lot of plant advice—I try my best—but one question is #1: how can I help the gardenia out? It was incredible last year.#2; The leaves turning red and dropping in some areas on the jasmine—hints? I don’t have a ton of time to devote and, if the advice is to repot, it’s not going to happen. I have a small patio and these are giant containers. And, yes, I’m a little lazy/over-scheduled, but my heart is in the right place. Going to Facebook to join your group now.

  4. Also some house plants do flower like anthuriums and Hoyas also have distinctive flowering here and there and love P. But its not needed in large amounts. We all know first shelf items are a no no

  5. Beautiful article. For years I used shelf synthetic fertilizers “plant food” which is 10-15-10. It did not help my plants bloom. I see a difference now, I use fish emulsion for my house tropical plants. 3-2-1 and it works well. I moved from synthetic fertilizers to more organic. I use fresh coco coir and somehow they obviously don’t have many nutrients because its a fresh coir block. I don’t pre soak so, I pot my plants, and add 3-2-1 fish emulsion and its sooooo good. My plants thrive and it doesn’t effect winter weather and dormancy. I guess its better for the roots and uptake of nutrients. But the soil is always 65-70F and air is 68-75F and humidity is about 60-80% some days when they are grouped together.

  6. This is a wonderful article. Top-dressing your garden with compost suffices as a natural fertilizer. There is rarely need for other fertilizers. I was wondering if compost could be used in place of mulch? Could it serve the same purpose or do you recommend bark mulch over the compost?

    • Sort of. It provides many of the benefits of mulch, but weeds grow in it, so it is not great for keeping weeds from growing.

      • I throw pine cones, fall leaves, twigs, wood chips, etc into my compost pile and all of that stuff doesn’t break down by the time I want to use the compost (unlike the more “green” inputs). So I just sift the compost and use the non-broken stuff as mulch, and it works fairly well if it’s thick enough.

  7. Hye Robert,great insight.Im guilty of fertilising my plants according to what needs I deem neccessary and so far have not seen much improvement by adding phosporus to induce blooming..If adding fertiliser is not needed,how do I induce blooming in plants that never seem to have any flowers,among them are my bleeding heart,adenium and bouganvillea.Thank you

    • Most flowering is induced by temperature and day length. Grow them with the conditions they like and they will flower once large enough.


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