Gibberellic acid is a natural plant hormone that can be used to speed up the germination of seeds. It is mostly used on seed that is difficult to germinate or ones that takes a long time to germinate. This post will examine how it works, and your options for using it.
What is Gibberellic Acid.
Gibberellic acid, or GA3 for short, is the most commonly used Gibberellin, of which there are about 100 different kinds. It is a natural plant hormone that affects plant growth.
An interesting application of GA3 is on Clementine Mandarin oranges. These oranges easily cross pollinate with other citrus fruits resulting in the production of seed. When GA3 is sprayed on the blossoms, clementines are produced without seeds.
Gibberellic acid can also have these effects.
- Overcome dormancy in seeds
- Cause premature flowering
- Increase fruit set
- Stimulate excess growth
- Offset the effects of frost on blossoms
- Inhibit the formation of roots on cuttings
As you can see it is a very interesting plant hormone, but in this post I will focus on germinating stubborn seed.
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Understanding Dormant Seeds
The development and germination of seeds is a complicated process and there is a lot of variation among species. Some species germinate easily and other species go into some type of dormant condition that prevents or delays germination. As gardeners, we use a number of techniques to release seeds from this dormancy and these are more fully described in my article, Seed Dormancy Explained.
To learn more about germinating seeds, have a look at my Youtube channel:
If the above video does not work, try this link: https://www.youtube.com/watch?v=dirz0WIMQi0&list=PLq7hmpP9i05Ska3k7gaBCvNCT9gN_tYaj
GA3 Helps Overcome Dormancy
GA3 is one technique that can be used to overcome some forms of dormancy. I’ll describe how to use it below, but in short you expose the seed to GA3 for a short period of time as a pretreatment at the beginning of the germination process. If it works, the seed will germinate faster.
There are some potential problems with this technique. If you use too much GA 3 (ie too high a concentration) or you expose the seed too long, you can kill the seed. It is also possible that you get good germination, but the seedlings grow too fast and become weak and elongated. And in other cases, it makes no difference to germination.
I generally use the Germination Guide of the Rock Garden and Hardy Plant Society for germination instructions on any specific species. If it suggests the use of GA3, it is worth trying. One problem with using this hormone is that there is limited information available for most species. It is a great opportunity for citizen science work.
Here are some examples of my experience with GA3.
Use of GA3 on Aquilegia Species
Some species of Aquilegia germinate easily at room temperature, but others are more difficult, especially once the seed has dried out. I tried using GA3 on several varieties obtained from seed exchanges, including Aquilegia flabellatta var. pumila, a species that has been very stubborn for me. GA3 did not make any difference. It is quite possible that the seed was too old, or the concentration was wrong. It is worth trying again with other Aquilegia species.
From Dr. Deno’s book, “Seed Germination, Theory and Practice”, (link provides a free copy), I learned that GA3 helps with old Aquilegia seed, but that it is not needed for fresh seed. This summer, I collected Aquilegia flabellatta var. pumila from my own plants and started germinating them as soon as they were ripe. I just left them sitting at room temperature and within a month they started to germinate.
For this species, GA3 is not needed if the seed is fresh, but may be helpful for older seed. For Aquilegia canadensis, soaking the seed in 500 ppm solution for 24 hours, speeds up germination.
Use of GA3 on Glaucidium Palmatum
In each of the past couple of years I was very fortunate to receive some Glaucidium palmatum seeds from the Ontario Rock Garden & Hardy Plant Society Seedex—thanks to everyone who donates seeds. The recommended procedure for germination is to plant as soon as they are ripe. That is not possible with our regular SeedEx since we don’t get the seed until it has dried for several months. I tried temperature cycling but that did not work. According to the ORGS Germination Guide and Dr. Deno’s book, G. palmatum is one species that does germinate easier with GA3. I decided to try some seed with GA3 and some without. The seeds were put into a baggy along with some Promix potting soil. They were all left at room temperature, some with room light, and some in the dark.
Contrary to some reports, darkness is not required for germination.
Seeds treated with GA3 started germinating 1 week after treatment and continued for a couple of months. Non-treated seed started germinating after about 2 months. The % germination was almost 100% with GA3 and around 50% without. None of the GA3 treated seedlings were deformed, and they all grew well.
Use of GA3 on Podophyllum Hexandrum
Podophyllum hexandrum (now called Sinopodophyllum hexandrum) are fairly difficult to germinate and usually require one or more cold/warm cycles. When the seed finally germinates it only produces a radicle (root) and in some cases it shows the cotyledon leaves. The first true leaf is not made until the seedling goes through another cold cycle which means it takes another year before you see a true leaf. In total it can take 2-3 years before you see the first true leaf after starting the seed.
Kristl Walek of Gardens North has reported that treatment with GA3 reduces the 2-3 years into one year.
I used seed from my own plants that had been dried and stored for 5 months. All of the seed was put into baggies along with moist peat moss, and left at room temperature for 3 weeks—nothing happened. The reason for this initial incubation is that I was waiting for my GA3 shipment. When the GA3 arrived, half the seed was treated and the other half was left untreated as a control.
The treated seed started to germinate within 2 weeks and continued with a few seeds germinating every week. Untreated seed did not germinate in the first 6 weeks, so I treated these seeds with GA3 and they started to germinate within a week of treatment.
Some seedlings started developing a true leaf 3-4 weeks after germinating. Others only showed the seed capsule above ground. For this latter group I dripped a few drops of a GA3 solution right onto the seed and radicle. Within a couple of weeks a true leaf started to grow.
None of the seedlings seemed to die or be spindly as a result of GA3 treatment. In a couple of cases the seedlings made two true leaves, which I think is unusual. Hopefully that translates into a stronger plant in future years.
In the case of Podophyllum hexandrum, GA3 reduced the long temperature cycling periods to a few weeks at room temperature, and it saved a years’ worth of growth.
GA3 for Clematis Seed
I reported previously on some work I did on clematis seed germination. GA3 hormone treatment was only applied to a few species where the literature suggests it would be helpful or required. Test results show that it is required or at least helpful for C. pitcheri and C. virginiana.
GA3 for Hard to Germinate Seed
The above are just a few examples where GA3 will help with germination. Many other seeds will also benefit.
An Article in the Bulletin of the American Rock Garden Society, Volume 49 Number 4, reported improvement in germination of these hard to germinate seeds; most Gentiana including Gentiana uerna, Campanula allionii, C. tridentata, C. cenisia, Phyteuma comosum, Edraianthus pumilio, Dionysia, Androsae, Soldanella, and Primula parryi,
Consider using GA3 on any seed that is reported to be difficult to germinate.
Best GA3 Treatment Method
GA3 can be purchased as a dry powder, which will keep for a very long time in the freezer. To use it on seeds, the powder needs to be dissolved in water so that seeds can absorb the chemical.
There are two common methods used. The Deno method was developed by Dr. Deno as part of his germination research. It is a simple method for homeowners to use, but it does not allow precise control over the concentration of GA3 delivered to the seeds. This is the method I use, and it seems to work quite well.
The prepared solution method, requires you to prepare a stock solution which can then be diluted to several concentrations as needed. The benefit of this method is that you control the concentration of GA3 and this can be important since too low a concentration does not work, and too high can damage the seed. The downside of this method is that it requires you to weigh the GA3 on an analytical balance, or buy the GA3 in premeasured packages, which is more expensive. The other problem is that the solutions have a limited shelf life.
Stability of GA3 in Solution
I can’t find a lot of good research to give a definitive answer here. In solution it will degrade faster than the dry powder. A solution is best stored cold and dark; a fridge works well for home use.
Some people suggest dissolving GA3 in an alkaline solution but studies show GA3 is not stable in such conditions. Use either distilled water or alcohol. Hard tap water can be slightly alkaline – so don’t use it.
Hudson Seeds claims to have tested solutions stored for up to 4 years at room temperature and found that they worked. There is no evidence they actually looked at the degradation on a chemical level or ran controls. I would certainly not store it at room temperature.
Gibberellic acid displays the greatest stability in solutions at pH 3-4. Neutral and weakly alkaline solutions are less stable. A few drops of vinegar might make solutions last longer.
Concentrations of GA3
A concentration of 1000 ppm GA3 is commonly used, but some people report excessive elongation of seedlings at this concentration. Others prefer to use 500 ppm or even 250 ppm. It is a good idea to test seed with more than one concentration to see which one works best.
The Deno Method
This method, along with pictures, is described in his book “Introduction to Seed Germination Theory and Practice” (link provides a free copy).
In short, you fold up a small piece of paper towel, place the seeds in it, along with a pinch of GA3, and add a few drops of water. Let sit for 24 hours in a plastic baggie. Then treat as normal for germination.
The amount of GA3 is controlled by using a double pointed toothpick which will result in about a 1000 ppm solution.
The Prepared Solution Method
Prepare a stock solution of 1000 ppm by dissolving 10 mg of GA3 in 10ml of water (or 100 mg/100ml). Kitchen measuring devices are not that accurate and it is best to use some lab glassware. You will also need some way to weigh out small amounts of GA3.
Since GA3 does not dissolve well in water, add a few drops of rubbing alcohol to the GA3 to dissolve it, and then add the required amount of water.
Other dilutions can be made from this stock solution. For example a 500 ppm solution is made by mixing 10 ml of the stock solution with 10 ml of water.
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