Stratification (botany)
Encyclopedia
In horticulture
, stratification is the process of pretreating seed
s to simulate natural winter conditions that a seed must endure before germination
. Many seed species undergo an embryonic dormancy
phase, and generally will not sprout until this dormancy is broken. The time taken to stratify seeds depends on species and conditions; though in many cases two months is sufficient.
is usually overcome by the seed spending time in the ground through a winter period and having its hard seed coat softened up by frost and weathering action. By doing so the seed is undergoing a natural form of "stratification" or pretreatment. This cold moist period triggers the seed's embryo
, its growth and subsequent expansion eventually break through the softened seed coat in its search for sun and nutrients.
stratification became the process by which seeds were artificially exposed to cold-moist conditions between layers of soil or peat to encourage subsequent germination in spring. Seed of many trees, shrubs and perennials require these conditions before germination will ensue.
In its most basic form, when the stratification process is controlled, the pretreatment amounts to nothing more than subjecting the seeds to storage in a cool (ideally +1° to +3°C; not freezing) and moist environment for a period found to be sufficient for the species in question. This period of time may vary from one to three months.
To accomplish this you merely place the seeds in a sealed plastic bag with moistened vermiculite
(or sand
or even a moistened paper towel) and refrigerate it. Use three times the amount of vermiculite
as seeds. It is important to only slightly dampen the vermiculite, as excessive moisture can cause the seeds to grow mouldy in the bag.
After undergoing the recommended period of stratification, the seeds are ready to be removed and sown in the nursery bed for germination. Alternatively, the seed may be sown in small pots filled with moist soil and then the whole thing enclosed inside a plastic bag before placing inside a common refrigerator.
pulp, leaf
and seed-pod fragments, cone
scales, etc.), but the shells of nuts
should not be removed.
Many sources recommend using peat
, a combination of peat and sand
, or vermiculite
as the medium for cold stratifying seeds. The medium must be sterile to prevent harm to the seed
by pathogens including fungi.
Soaking the seeds in cold water for 6–12 hours immediately before placing them in cold stratification can cut down on the amount of time needed for stratification, as the seed needs to absorb some moisture to enable the chemical changes that take place.
Any seeds that are indicated as needing a period of warm stratification followed by cold stratification should be subjected to the same measures, but the seeds should additionally be stratified in a warm area first, followed by the cold period in a refrigerator later. Warm stratification requires temperatures of 15-20°C. In many instances, warm stratification followed by cold stratification requirements can also be met by planting the seeds in summer in a mulched bed for expected germination the following spring. Some seeds may not germinate until the second spring.
to moisten your stratifying vermiculite will help prevent fungal diseases. Chinosol (8-hydroxyquinoline sulfate), primarily a disinfectant and often recommended for growing succulents from seed prone to mold, is one such fungicide.
Different seeds should be placed in different bags rather than putting them all into one bag, and large quantities are also best split into several small bags. That way any fungal outbreak will be restricted to only some seeds. If no fungicide is used, a close check should be kept on the seeds, removing any which show signs of mould or become soft and with a decaying smell.
If an outbreak of fungus occurs, remove the seeds and re-apply fungicide, then place them in a new bag with new slightly moistened vermiculite
. Always keep the bag sealed. The stratifying seeds should be checked on a regular basis for either fungus or germination. If any seeds germinate while in the refrigerator, they should be removed and sown.
s, whether grown in pots or beds, benefit from good air circulation which discourages fungus growth and promotes sturdy stems. Potting and germinating medium/soil is not critical as long as the soil is light as well as lightly firmed down but not heavily compacted. Sterilised potting soil will minimize problems with Botrytis
or Pythium
fungal disease. These problems are much more likely to occur if air circulation is poor.
Most seeds need only be planted at a depth equal to their own thickness in order to germinate. Seeds planted outdoors are best planted little deeper to avoid disturbance caused by heavy rainfall. The soil should be slightly damp but never soaking wet, nor allowed to dry out completely.
Horticulture
Horticulture is the industry and science of plant cultivation including the process of preparing soil for the planting of seeds, tubers, or cuttings. Horticulturists work and conduct research in the disciplines of plant propagation and cultivation, crop production, plant breeding and genetic...
, stratification is the process of pretreating seed
Seed
A seed is a small embryonic plant enclosed in a covering called the seed coat, usually with some stored food. It is the product of the ripened ovule of gymnosperm and angiosperm plants which occurs after fertilization and some growth within the mother plant...
s to simulate natural winter conditions that a seed must endure before germination
Germination
Germination is the process in which a plant or fungus emerges from a seed or spore, respectively, and begins growth. The most common example of germination is the sprouting of a seedling from a seed of an angiosperm or gymnosperm. However the growth of a sporeling from a spore, for example the...
. Many seed species undergo an embryonic dormancy
Dormancy
Dormancy is a period in an organism's life cycle when growth, development, and physical activity are temporarily stopped. This minimizes metabolic activity and therefore helps an organism to conserve energy. Dormancy tends to be closely associated with environmental conditions...
phase, and generally will not sprout until this dormancy is broken. The time taken to stratify seeds depends on species and conditions; though in many cases two months is sufficient.
In the wild
In the wild, seed dormancySeed dormancy
Seed dormancy is a condition of plant seeds that prevents germination when the seeds are under optimal environmental conditions for germination. Living, non dormant seeds germinate when soil temperatures and moisture conditions are suited for cellular processes and division; dormant seeds do...
is usually overcome by the seed spending time in the ground through a winter period and having its hard seed coat softened up by frost and weathering action. By doing so the seed is undergoing a natural form of "stratification" or pretreatment. This cold moist period triggers the seed's embryo
Embryo
An embryo is a multicellular diploid eukaryote in its earliest stage of development, from the time of first cell division until birth, hatching, or germination...
, its growth and subsequent expansion eventually break through the softened seed coat in its search for sun and nutrients.
Process
Stratification is the process of subjecting seeds to both cold and moist conditions. Typically, temperatures must be between 34° and 41°F. The term can be traced to at least 1664 in Sylva: or a discourse of forest trees & the propagation of timber, Vol. II. where seeds were layered (stratified) between layers of moist soil and exposing these strata to winter conditions. Thus,stratification became the process by which seeds were artificially exposed to cold-moist conditions between layers of soil or peat to encourage subsequent germination in spring. Seed of many trees, shrubs and perennials require these conditions before germination will ensue.
In its most basic form, when the stratification process is controlled, the pretreatment amounts to nothing more than subjecting the seeds to storage in a cool (ideally +1° to +3°C; not freezing) and moist environment for a period found to be sufficient for the species in question. This period of time may vary from one to three months.
To accomplish this you merely place the seeds in a sealed plastic bag with moistened vermiculite
Vermiculite
Vermiculite is a natural mineral that expands with the application of heat. The expansion process is called exfoliation and it is routinely accomplished in purpose-designed commercial furnaces. Vermiculite is formed by weathering or hydrothermal alteration of biotite or phlogopite...
(or sand
Sand
Sand is a naturally occurring granular material composed of finely divided rock and mineral particles.The composition of sand is highly variable, depending on the local rock sources and conditions, but the most common constituent of sand in inland continental settings and non-tropical coastal...
or even a moistened paper towel) and refrigerate it. Use three times the amount of vermiculite
Vermiculite
Vermiculite is a natural mineral that expands with the application of heat. The expansion process is called exfoliation and it is routinely accomplished in purpose-designed commercial furnaces. Vermiculite is formed by weathering or hydrothermal alteration of biotite or phlogopite...
as seeds. It is important to only slightly dampen the vermiculite, as excessive moisture can cause the seeds to grow mouldy in the bag.
After undergoing the recommended period of stratification, the seeds are ready to be removed and sown in the nursery bed for germination. Alternatively, the seed may be sown in small pots filled with moist soil and then the whole thing enclosed inside a plastic bag before placing inside a common refrigerator.
Preparing a stratifying medium
The seeds should be cleaned of any additional material (fruitFruit
In broad terms, a fruit is a structure of a plant that contains its seeds.The term has different meanings dependent on context. In non-technical usage, such as food preparation, fruit normally means the fleshy seed-associated structures of certain plants that are sweet and edible in the raw state,...
pulp, leaf
Leaf
A leaf is an organ of a vascular plant, as defined in botanical terms, and in particular in plant morphology. Foliage is a mass noun that refers to leaves as a feature of plants....
and seed-pod fragments, cone
Conifer cone
A cone is an organ on plants in the division Pinophyta that contains the reproductive structures. The familiar woody cone is the female cone, which produces seeds. The male cones, which produce pollen, are usually herbaceous and much less conspicuous even at full maturity...
scales, etc.), but the shells of nuts
Nut (fruit)
A nut is a hard-shelled fruit of some plants having an indehiscent seed. While a wide variety of dried seeds and fruits are called nuts in English, only a certain number of them are considered by biologists to be true nuts...
should not be removed.
Many sources recommend using peat
Peat
Peat is an accumulation of partially decayed vegetation matter or histosol. Peat forms in wetland bogs, moors, muskegs, pocosins, mires, and peat swamp forests. Peat is harvested as an important source of fuel in certain parts of the world...
, a combination of peat and sand
Sand
Sand is a naturally occurring granular material composed of finely divided rock and mineral particles.The composition of sand is highly variable, depending on the local rock sources and conditions, but the most common constituent of sand in inland continental settings and non-tropical coastal...
, or vermiculite
Vermiculite
Vermiculite is a natural mineral that expands with the application of heat. The expansion process is called exfoliation and it is routinely accomplished in purpose-designed commercial furnaces. Vermiculite is formed by weathering or hydrothermal alteration of biotite or phlogopite...
as the medium for cold stratifying seeds. The medium must be sterile to prevent harm to the seed
Seed
A seed is a small embryonic plant enclosed in a covering called the seed coat, usually with some stored food. It is the product of the ripened ovule of gymnosperm and angiosperm plants which occurs after fertilization and some growth within the mother plant...
by pathogens including fungi.
Soaking the seeds in cold water for 6–12 hours immediately before placing them in cold stratification can cut down on the amount of time needed for stratification, as the seed needs to absorb some moisture to enable the chemical changes that take place.
Any seeds that are indicated as needing a period of warm stratification followed by cold stratification should be subjected to the same measures, but the seeds should additionally be stratified in a warm area first, followed by the cold period in a refrigerator later. Warm stratification requires temperatures of 15-20°C. In many instances, warm stratification followed by cold stratification requirements can also be met by planting the seeds in summer in a mulched bed for expected germination the following spring. Some seeds may not germinate until the second spring.
Use of fungicide
Use of a fungicideFungicide
Fungicides are chemical compounds or biological organisms used to kill or inhibit fungi or fungal spores. Fungi can cause serious damage in agriculture, resulting in critical losses of yield, quality and profit. Fungicides are used both in agriculture and to fight fungal infections in animals...
to moisten your stratifying vermiculite will help prevent fungal diseases. Chinosol (8-hydroxyquinoline sulfate), primarily a disinfectant and often recommended for growing succulents from seed prone to mold, is one such fungicide.
Different seeds should be placed in different bags rather than putting them all into one bag, and large quantities are also best split into several small bags. That way any fungal outbreak will be restricted to only some seeds. If no fungicide is used, a close check should be kept on the seeds, removing any which show signs of mould or become soft and with a decaying smell.
If an outbreak of fungus occurs, remove the seeds and re-apply fungicide, then place them in a new bag with new slightly moistened vermiculite
Vermiculite
Vermiculite is a natural mineral that expands with the application of heat. The expansion process is called exfoliation and it is routinely accomplished in purpose-designed commercial furnaces. Vermiculite is formed by weathering or hydrothermal alteration of biotite or phlogopite...
. Always keep the bag sealed. The stratifying seeds should be checked on a regular basis for either fungus or germination. If any seeds germinate while in the refrigerator, they should be removed and sown.
Sowing and seedlings
Most seedlingSeedling
thumb|Monocot and dicot seedlingsA seedling is a young plant sporophyte developing out of a plant embryo from a seed. Seedling development starts with germination of the seed. A typical young seedling consists of three main parts: the radicle , the hypocotyl , and the cotyledons...
s, whether grown in pots or beds, benefit from good air circulation which discourages fungus growth and promotes sturdy stems. Potting and germinating medium/soil is not critical as long as the soil is light as well as lightly firmed down but not heavily compacted. Sterilised potting soil will minimize problems with Botrytis
Botryotinia
Botryotinia is a genus of ascomycete fungi causing several plant diseases. The anamorphs of Botryotinia are mostly included in the imperfect fungi genus Botrytis...
or Pythium
Pythium
Pythium is a genus of parasitic oomycete. Most species are plant parasites, but Pythium insidiosum is an important pathogen of animals...
fungal disease. These problems are much more likely to occur if air circulation is poor.
Most seeds need only be planted at a depth equal to their own thickness in order to germinate. Seeds planted outdoors are best planted little deeper to avoid disturbance caused by heavy rainfall. The soil should be slightly damp but never soaking wet, nor allowed to dry out completely.