Double Dormancy In Seed – Does It Exist?

In a previous post, Seed Dormancy – Are Seeds Really Dormant?, I discussed Seed Dormancy and presented a new way for gardeners to look at seed development and seed dormancy. Today I would like continue the discussion by looking at double dormancy and ask the question, does it really exist?

If you plant some peony or trillium seed in the fall you won’t see any green growth until the second spring – if you’re lucky. This is routinely described as an example of double dormancy – the seed needs two cold periods before they germinate. The two stratification (cold) periods overcome two dormancies, hence the name, double dormancy.

But is this really true? Do seeds like peony and trillium have have double dormancy? Does any seed have a double dormancy?

Peony Seed Germination

Let’s have a close look at how peony seed develops. This applies to all of the herbaceous peonies and tree peonies that I have germinated and it seems to apply to most species.

After seed maturation, the seed is hard and dry. It is large and quite easy to handle. A common germination procedure is to put the seed into a baggy (clear plastic sandwich zip-lock bag) with moist peat moss or vermiculite. Then leave the baggy  sit at room temperature.

The baggy allows you to observe any development that takes place.

After several months, the seed coat breaks and a radicle (root) emerges. This root will elongate to several inches and start making side roots. After the root gets to a certain size, growth slows to almost a stop. Nothing more happens. No green shoot is produced. The seed can sit this way for many months with no visible sign of change.

If you take the sprouted seed and cool it down to 5 deg C, a shoot will develop in a couple of weeks. You can now pot up the new seedling and grow a mature plant.

Trillium Seed Germination

This discussion applies to Trillium grandiflora, Trillium errectum and most other species. My experience is only with the former two.

Fresh collected seed that is kept moist will have a certain % germination after a few months time at room temperature. A root and tuber will develop but no leaves will form. It is only after the plant spends some time at 5 deg C, that a shoot starts to grow. For more detail on this and some interesting seed/germination pictures have a look at the Botancially Inclined website.

You will note that in the above description, only some of the collected seed follows this process. The remaining seed are dormant and require a cold period before any radicle starts to grow. In nature these seed will develop the root in the second summer, and then the shoot in the following summer after another cold period.

If the fresh seed is allowed to dry out, then the seed enters a deep dormant stage which will require a cold treatment before the appearance of the radicle. If it dries too much the dormancy is so deep that the seed is dead.

Some fresh, moist trillium seed germinates with no cold period. The rest of the seed, and dry stored seed requires a cold stratification period before roots develop followed by a second cold period before a shoot is formed.

What is Germination

Before we can determine if these seeds have double dormancy it is necessary to define the term ‘germination’.

The term has been used in at least 3 different ways.

Wikipedia defines germination as ” a process by which a plant grows from a seed” (ref 1).  This is a common definition found on many web sites.

This is a simple and easy to understand definition, but it’s not of much use. How much of the seedling growth or mature plant growth is part of germination? When is germination finished? This definition is too general to be of any practical use.

A scientific definition goes as follows, “germination incorporates those events that commence with the uptake of water by the quiescent dry seed and terminates with the elongation of the embryonic axis (ie root and shoot development)” (ref 2).

The definition is also unclear. How is ‘elongation of the embryonic axis’ defined? The embryonic axis includes both the root and the shoot. Elongation can occur inside the seed coat with nothing showing outside the seed – is this enough for germination to have occurred? Do they need to show themselves outside of the seed coat? If so, do both root and shoot need to show themselves or just one of them?

This definition has been interpreted as “germination is completed by the penetration of the structures surrounding the embryo by the radicle; the result is often called visible germination” (ref 7).

This is similar to the most commonly found definition used by both hobbyists and scientists including Dr Norman Deno, a pioneer in seed germination studies. Germination is the point at which the radicle or shoot first appears outside of the seed coat. I don’t know of any case where the shoot appears before the root, but such a case would also qualify.

This last definition seems to be the most practical for gardeners and it defines a specific, well defined event in the development of a plant.

Note that none of the above definitions require the shoot to show itself above ground.  In a moment you will understand why I am making this point.

What Is Double Dormancy?

Many people believe that double dormancy exists when seed requires two cold periods, separated by a warm period, in order for germination to take place. This definition is incorrect. A requirement for two cold periods could qualify as double dormancy, but it is not the only set of conditions that qualify.

Another definition that I found on the Colorado State University Extension website is that “double dormancy exists if there is both seed coat dormancy (impervious seed coat) and embryo dormancy (requires time or temperature changes to develop)” (ref 5). This again is one possible situation which meets the requirements of double dormancy, but it is not a definition of all cases.

Double dormancy is defined as a condition where seeds need to overcome two or more primary dormancies in order to germinate (ref 4). Note the words “two or more” which seems odd given the name “double” dormancy.

Examples of double dormancy include:

• Seed that needs to have the seed coat damaged so water can be absorbed, and then it needs a cold period before radicle growth.
• Seed that needs a warm period to complete embryo development, and then needs a cold period for radicle development.

Do Peonies Have Double Dormancy?

Historically peonies have been labeled as having double dormancy, because it takes two winters before growth is seen above the ground. As explained above, germination does not require a part of the plant to show itself above ground. The requirement of ‘two years’ is NOT a reason to label these seeds double dormant.

Peony seed germination does not require two cold periods.

Peony seed will not germinate immediately. When seed is ripe and collected from the plant, the embryo is still not fully developed. It requires a few months in warm conditions to finish develop. There may also be a slight seed coat dormancy, but that is not clearly established and therefore I will ignore it in this discussion.

Once roots develop they need to reach a certain size and then get a cold period before the shoot starts to develop. Interestingly, 85% of the seeds develop a shoot after chilling if the radicle is 4 cm long, but only 40% will do so if the radicle is only 2-3 cm long (ref 4). Size does matter! Some studies show that the radicle should be at least 6 cm long before cold treatment.

Do peonies show double dormancy?

Based on the definition of germination I provided above, namely that germination occurs as soon as either the root or the shoot emerge from the seed, then it is clear that there is NO double dormancy. The only dormancy that needs to be over come is that of an immature embryo ie embryo dormancy. The root does not need a cold period before it shows itself outside of the seed.

What about the dormant shoot? This is definitely a dormancy, but it is a dormancy of plant growth and takes place after germination. Adult peonies have similar annual dormancy periods.

Do Trillium Exhibit Double Dormancy?

Here is a quote I found, “Trillium flexipes has double dormancy, in which two seasons of cold are required for germination of seeds (Pringle 1984). In the first year, only roots develop from the seed and it is not until the second year that cotyledons emerge above-ground”.

This is a good example of why the information on this subject is so confusing. The quote says Trillium flexipes is double dormant and then go on to say it germinates the first year – contradicting themselves.

A percentage of fresh trillium seed will follow the same germination pattern as peonies – provided they are kept moist. Using the common definition of germination trillium does NOT exhibit double dormancy.

However, trillium is more complicated than that. Some of the fresh seed will not germinate until after a cold period. This seed has both an embryo dormancy as well as a root dormancy, so it clearly exhibits double dormancy.

Dried trillium seed is similar and therefore also shows a double dormancy.

Does Double Dormancy Exist?

Double dormancy for germination definitely exists. Many types of seed have complex dormancies that need to be overcome before germination takes place.

Many other seeds have a simple dormancy and germinate easily.

Between these extremes we find many other seeds that may or may not exhibit double dormancy, depending on the definition of germination. If we accept the definition of germination as the emergence of either a root or shoot, than many of the seeds that are labeled as being double dormant are in fact not double dormant. Many of the temperate North American spring flowering natives fall into this category.

The idea that the development of the shoot in trillium is a plant dormancy, and not a seed/germination dormancy is not a very commonly expressed idea but I believe it is the only reasonable way to explain things. The way trillium shoots grow from the seed is very similar to annual trillium growth of the mature plant in mid and late summer (ref 6). Both need a cold period to initiate growth. Both are developing leaves so that they are ready to emerge above ground in early spring. Both have similar dormancies.

If you don’t accept this idea, then you must to go back and re-define germination as requiring the emergence of both a root and a shoot. Science could do that, but they haven’t. Unfortunately, in discussions of double dormancy, scientists do not separate seed dormanices from seedling (ie plant) dormancies, as I have tried to do here.

Seed double dormancy does exist, but the term is incorrectly applied to a number of plants.

References:

1. Wikipedia definition of germination: https://en.wikipedia.org/wiki/Germination
2. Seed Dormancy and Germination: http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3243337/
3. Seed Dormancy In Eastern Redbud:
4. Seed Dormancy in Commercial Vegetable and Flower Species: https://www.uky.edu/Projects/SeedBiology/research/DORMAN.PDF
5. Plant Structures – Seeds: http://www.ext.colostate.edu/mg/gardennotes/137.html
6. The Dark Side of Trillium:
7. Seed Germination and Dormancy: http://www.ncbi.nlm.nih.gov/pmc/articles/PMC156979/