Plant Starters and Root Boosters – Do They Work?

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Robert Pavlis

It’s planting time and most fertilizer manufacturers and nurseries are pushing their high phosphorus products, usually under names like Plant Starter, Root Booster and the one I really like “MegaMass”. These fertilizers claim to “supply the high phosphorus needed for rapid root development”.

Who doesn’t want good roots on their newly planted babies? Are roots not the key to great plants?

Let’s have a look at this extremely common myth.

Plant Starters and Root Boosters - Do They Work?. photo source: Farmer's Almanac
Plant Starters and Root Boosters – Do They Work? ;source: Farmer’s Almanac

Why Do People Sell High Phosphorus Fertilizer?

Some poorly designed studies in the 1940s and 1950s found that phosphorus stimulated root growth. At the time, most plants were propagated in soil which absorbs excess phosphorus and makes it unavailable to plants. Adding higher amounts did overcome this problem in a limited way. More studies followed and it is now well understood that this is a very short term bandage.

Fast forward to today, when almost all propagation is done in a soilless mix and the problem of absorbed phosphorus is no longer an issue. With today’s horticultural practices, there is no advantage to using high phosphorus fertilizers – they do not produce larger root systems.

Even when these plants are transplanted into real soil, an excess amount of phosphorus does not stimulate root growth because it is quickly immobilized (no longer plant available) by soil.

Building Natural Ponds book, by Robert Pavlis

After those early studies, fertilizer manufacturers started making high phosphorus fertilizers. NPK values of 10-52-10 became popular and they are still being promoted today as plant starters and root boosters. There is no science to support the use of these products.

What is the Ideal NPK?

Numerous studies have shown that an NPK ratio of 3-1-2 or 3-1-3 works well for most plants.

As an example, this study looked at the growth of Lantana and found a “high” phosphorus level did not produce better plants. What did they consider high? High for them was 3/4 of the nitrogen level. A 10-52-10 fertilizer has 5 times the nitrogen level. They would consider that toxic!

Michigan State University developed the “perfect” fertilizer for a wide variety of plants and their ratio is 5-1-5.

Most commercial plant growers are using fertilizer in the range of 3-1-2.

A study called “Do Plants Require Nutrients in Similar Proportions“, looked at a large number of plants and studies and concluded that a good average NPK ratio is 3-1-2.5.

No Support for a Balanced Fertilizer

Many people recommend using a balanced fertilizer, like 10-10-10. That is also promoted by fertilizer companies with no scientific support.

Anyone who makes this recommendation is just following marketing nonsense.

“Balanced Fertilizers (e.g. 10-10-10, 15-15-15, 20-20-20, etc.) induce stretching in plants. Plants become too tall and spindly, leading to increased breakage and undesirable appearance. ”

Environmental Damage

Reports suggest that global phosphorus reserves are dwindling and that “the global reserve will be depleted within the next 50 to 100 years.” I don’t know how true that is but what is clear is that excess phosphorus does find its way into streams and lakes where it causes pollution and algae growth.

Compost Science for Gardeners by Robert Pavlis

Spreading excess phosphorus on soil is harming the environment. So don’t do it.

High Phosphorus Inhibits Mycorrhizal Fungi

Mycorrhizal fungi can be important for plants. It is well known that high phosphorus levels inhibit their association with plant roots.

YouTube video

Bloom Boosters

Bloom boosters are also popular and these also contain high phosphorus levels. To be honest a bloom booster is just a plant starter that is sold under a different label.

Bloom boosters do not work. They do not increase the number of flowers on a plant – just more marketing nonsense.

Which Fertilizer Should You Use?

For seedlings and potted plants, in a soilless mix, use a fertilizer with a ratio of 3-1-2.

When you plant in real soil you normally do not need to add fertilizer. If you add compost and manure you definitely don’t need fertilizer, unless a soil test tells you differently.

For vegetable gardens you might add some extra nitrogen, but no phosphorus.

To understand the difference between a 3-1-2 and a 6-2-4 fertilizer, have a look at Fertilizer NPK Ratios – What do they Really Mean?

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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!

11 thoughts on “Plant Starters and Root Boosters – Do They Work?”

  1. Thanks for this- when learning about growing plants, one is surrounded by an abundance of advice, and it’s easy to assume that people know what they are talking about. As your blog makes clear, so much gardening advice is untested and unquestioned factoids with little empirical- or even logical- backing.

  2. Great to get reliable information. Why don’t all product clains have to be supported by reliable data?

  3. Michigan university link of 5:1:5 fertilizer points to AOS website..and seems only applivable to orchid. Can you please post correct reference

  4. Can’t wait to print this and give it to my father (90yo) he thinks the more fertilizer the better. I grow all my plants and follow your advice and have no problems at all.

  5. Twitter & facebook banned Mr Trump for telling a mere 32,000 lies. I wonder what the standard is for garden products?


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