Rooting Hormones – What Are They?

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Robert Pavlis

Rooting hormones are recommended for rotting plant cuttings. To help you decide if you need them, and which ones to select, it is useful to know more about them. In this post I will provide back ground information about rooting hormones. In future posts I’ll show you how to use them.

Rooting Hormones - What Are They
Rooting Hormones – What Are They

Why Use Rooting Hormones?

The name ‘rooting hormone’ sounds like it would be a good thing to add to your water before watering plants. Who doesn’t want better roots. Although they might do some good on established plants, such plants already produce their own rooting hormones. You should not add more when you water.

Rooting hormones are used for plant propagation – growing from cuttings in particular.

Many plants can be propagated (ie multiplied) by taking a piece of the mother plant, and using it to produce a new plant. In some cases, like African violets, all you need is a leaf, and you can grow a new plant. For many perennials you can take leaf or stem cuttings and grow them into plants. Using leaves does not work for trees and shrubs, but in this case you can take part of the stem called a cutting, and grow it.

I’ll discuss propagation methods more in future posts, but the general procedure is as follows. Take part of the plant, and put it in soil. Maintain a high humidity and wait. In a few weeks or a few months the piece of plant will have roots, and once roots form, the plant will make new leaves. A new plant is born.

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The key to making this work, is getting the roots to grow. Without roots, the plant won’t grow. Some plants make roots very easily. Some succulents, for example, make roots on every leaf that falls to the ground. Other plants, especially woody plants make roots much more reluctantly.

In order to help plants make roots, you can add rooting hormones to the pieces of plant material before putting them in soil. In this post I’ll discuss these rooting hormones.

Plant Hormones

You probably know that humans have natural hormones in their body. We talk about raging hormones in teenagers, and the pros and cons of hormone replacement therapy. Melatonin is a hormone that controls how we feel when we are awake.

Plants also have hormones and these chemicals are called plant hormones. These are different than the hormones found in animals.

Plant hormones fall into five main classes:

  • Auxins are produced in terminal buds and suppress the growth of side buds and stimulate root growth.  They also affect cell elongation (tropism), apical dominance, and fruit drop or retention.
  • Gibberellins affect cell division, flowering, size of fruit and leaves, and they are important in overcoming seed dormancy.
  • Cytokinins promote cell division, aging in leaves, and influence cell differentiation
  • Abscisic acid is a stress hormone that inhibits other hormones during periods of stress.

 

As you can see hormones are critical for the growth and development of plants. The hormones that will be discussed in this post are the Auxins, which are responsible for root growth.

Auxins – the Rooting Hormones

There are two naturally occurring auxins.

  • Indole-3-acetic acid (IAA)
  • Indole-3-butyric acid (IBA)

IAA is the most abundant auxin in plants, but it is not used very much for propagation since it breaks down quickly in plants and when exposed to light. IBA is found in very small amounts in plants because plants convert it to IAA.

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Man has also synthesized compounds that act like auxins in plants and these include:

  • alpha-Naphthalene acetic acid (NAA)
  • 2,4-diclorophenoxyacetic acid (2,4-D),

The herbicide, NAA is a molecule that is very similar to IAA, and mimics IAA in plants – plants can’t really tell the difference between the two. The chemical 2,4-D is used for tissue culture work but is usually not used as a rooting hormone.

Commercial Rooting Products

The auxins help plants produce roots. It is only natural that someone creates products that can be applied to plants to help them make roots – commercial rooting products. Products like Clonex, a brand name, are called a number of different things, like; rooting gel, rooting compound, cloning gel, rooting solution, rooting powder and rooting medium. These are all essentially the same thing – rooting hormones.

IBA and NAA are not soluble in water. They will dissolve in alcohol and other solvents, but these solvents can harm plant cuttings. The potassium salts of IBA nad NAA are water soluble and are more commonly used in commercial products. Most of the time when people refer to IBA and NAA, they are actually talking about the potassium salt, not the pure compounds. I’ll also will follow this convention.

IBA and NAA are the two hormones that are most effective in producing roots on cuttings.

Hormone Concentrations

Think of hormones as very powerful chemicals. A very small amount can have a very large effect. Not enough, and there is no effect on the plant. Too much and the plant is harmed. It is like any medicine we take – you have to get the dose right.

For herbaceous and softwood cuttings, rooting hormones are applied in concentrations of 500-1,500 ppm. Rates between 1,000 and 5,000 ppm are used for semi-hardwood cuttings and 10,000 ppm is used for hardwood cuttings. Plant parts that are harder to root need more hormone.

When the correct amount is added to the cutting, the cutting will show some swelling, form callus tissue, and then form roots.

If too much rooting hormone is used it damages the cutting, resulting in yellowing leaves, blackening of the stem and even death of the cutting. Some products that are dissolved in alcohol are more likely to burn or dehydrate the cutting.

It is important to follow directions when applying the hormones.

References:

  1. Photo source: Chiot’s Run
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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!

10 thoughts on “Rooting Hormones – What Are They?”

  1. I wander if a % of rooting success rate -with and without hormones- has been made? I am growing lavenders in the tropics, initially from seeds and then from cuttings when they are well developed. The bottle of hormone I bought from Amazon disappeared a few weeks after I received it so I really could not get significant data, but the general impression was not impressive. However, I am now very conscious about the effect of the amount of hormone since I usually am the more-is-better type.
    So I am back to hormone-less growing (several hundreds so far). Results are following (remember: in the tropics):
    1) green tender cuttings are about twice as successful as woody hard ones.
    2) global success rate is about 50%.
    3) about a month is necessary to see if a cutting has rooted. After a month I normally discard.
    4) blooming may take place after 9 months and continues all year round.

    Question:
    Will rooting hormone increase the success rate?
    Any information about rooting lavender specifically?

    This site has proved very useful. Congratulations.

    Reply
  2. HI Robert
    Can we use rooting hormones on established plants to have more development of roots? If yes, how to use ? Spray or Drench ?

    Reply
  3. Question my sister rooted a Rex B. In water it has some roots but I want to transfer to soil. Can I put root hormone on the roots already formed?

    Reply
    • Rooting hormones have nothing to do with the sickness of a plant, or how long they live. Using too much rooting hormone at rooting time can prevent the formation of roots.

      Reply
  4. Excellent post and well thought out. Yes, I’ve sometimes heard of people purchasing some type of B-1 with hormones infused into the concoction thinking they could increase the health of an established plants root system. It’s totally unnecessary and waste of money as it won’t really accomplish what they purposed or imagined.

    I’ve never spent much time with cutting propagation, but have done it. Back in 1986 I once collected cuttings from some upright branch tips of Big-Cone Douglas Fir (Pseudotsuga macrocarpa) which is native only to Southern California. For most folks it’s a tree of mystery because it is so often isolated on steep slopes within chaparral plant community and borderline of where the normal forest line starts. Often inaccessible and when it is the seed cones are way high up on the tree and at the tips of branches. I have in all my 30 years of collecting seeds never acquired viable seed from any cones.

    However the common talc powder rooting hormone worked great and you are correct with not over using it. The average gardener has this inner voice which tells them, “If a little works, more must be better.” I always found this true with the product you well know called “Super Thrive”. I only used it in the 1970s and early 1980s. A bottle if used as directed last some couple years. If one followed the actual directions with just a couple drops to a gallon, then transplants would do well and start growing immediately from the start, but use cap fulls to a gallon and nothing happened. The plant would stay stuck in neutral. I have not however used that product in decades since and think it’s a waste of money, although it’s not terribly expensive. Still, most people will never understand how to use it.

    But the amazing effect of talc powder on my Big Cone Doug fir cuttings was amazing. As usual, the cutting ends swelled and turned into swollen bumpy nodules from which the first roots emerged. My cuttings I collected in later winter just before the buds on the branch tips started to swell for bud break. Interestingly after a month, the branch tips did indeed sprout and grew about an inch or more, but the root also emerged some weeks later. It was kool because the US Forest Service has never made any attempt to propagate and re-establish this tree which has incredible tolerance for extreme heat and drought. It is also on of the few trees that will sprout back on all it’s branches and trunk from wildfire. I tried to share the experience with friends up in Idyllwild Forestry station and they were intrigued, but it never went beyond mild interest. Sadly I out planted them back into the wild, but even myself never pursued them any further. But I really should have though as I think they would have made an excellent landscape tree for urban settings.

    Reply
      • You know Robert, it may be for the reasons they have abilities to sprout back from fires. This may likewise be the case also for Canary Island Pine (Pinus canariensis), but I never thought of cloning any Canary Island Pine bud cuttings. I’d love to though as it would be interesting. I go to the Canary Islands quite a bit and it is amazing to see hundreds of acreage of formerly burned forest with every tree in re-sprout. It is a long history of volcanism and maybe this is why the trees have so engineered themselves.

        Reply

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