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Treatments in seeds

The aim of every seeds is the germination, a process that transforms the embryo contained in seeds, into a new plant. For that to happen, the seed needs to be in the presence of the following factors:

  • appropriated temperature
  • humidity
  • gaseous environment (breathing)

In most of cases, seeds germinate immediately after getting rid of plants. But some others, especially tree an bush species, have developed that ability to control the start of germination, so that to coincide with the time of the year having the favourable natural conditions for plantling survival. This intern mechanism is called lethargy, or sleeping. For example, seeds that disseminate at the beginning of winter like cherry tree or prune tree, strictly need to go through hot period (summer) followed of cold period (autumn and winter) to germinate. It means that till spring of the following year, dissemination won’t be in good conditions to germinate. Other species needs the covering to be deteriorated and round up. In concrete terms, they have to pass through stomachs of birds that eat fruits.

Everyone who wants to make germinate a lethargic seed will have to try to approach the main measurement of natural germination process, with previous treatments of sow. According to the cause of seeds lethargy, an appropriated treatment will be applied.

The different kinds of lethargy existing are:

 1.     EXOGENEOUS LETHARGY

Physical

Causes: impermeable covering

Treatment: scarification

Example: Robinia pseudoacacia

Chemical

Causes: inhibitor of covering

Treatment: to separate coverings

Mechanical

Causes: mechanical resistance of coverings to the embryo development.

Treatment: to break coverings

Example: Elaeagnus angustifolia

 2.     ENDOGENEOUS LETHARGY

Morphologic

Causes: embryo underdevelopment

Treatment: hot stratification

Example: happens when combined with other factors

Physiologic: soft, intermediate or deep

Causes: physiologic inhibition of germination mechanisms

Treatment: cold stratification

Example: Sorbus aucuparia

Morpho-physiologic

Causes: combination of embryo underdevelopment and physiologic inhibition

Treatment: hot stratification followed with cold stratification

Example: Fraxinus excelsior

Next we will explain without going into details the most common pre-sowing treatments in seeds use.

TYPES OF SEEDS TREATMENTS 

1. DIRECT SOWING: they are called « direct sowing” because there is no need of specific pre-germination treatment. In general, it’s advisable to submerge seeds in water before sowing (24 to 48 hours). Seeds with very hard layer need to spend many weeks in water before sowing them. For the rest of them, the general advices written forward are valid.

2. STRATIFICATION: slow rehydration process, advisable in almost every case. This process allows a better germination and growth in term of homogeneity.

Method: let seeds in medium-temperature water for 24 to 48 hours. Prepare the place of stratification (mix of fine sand or vermiculite with turbe: 50/50 or 1/3+2/3), disinfecting it before. Add to this mix the drained seeds, moisten the set and keep it in a closed plastic bag, with the correct temperature indicated for every species.

For some species like Prunus avium or Fagus sylvatica, stratification can be made without natural environment, just by putting hydrated seeds in an hermetic closed bag.

During the stratification time, it is very important to keep an eye on the environment moisture, to add some water if it does require, and to control the start of germination, in which case sowing will have to be brought forward.

Duration of stratification

According to species, stratification duration can late from 1 month (especially with coniferous), to many months (a big part of lush seeds), till 1 or 2 years (Taxus, Tilia, ...). So the stratification starting date will be calculated according to the remaining period from the stratification date to the sowing date (D. beginning stratification = D. sowing - stratification time). In any case, the more advisable sowing period won’t be after April.

Stratification types
Some species require a doble stratification, which means, a hot stratification time followed by a cold stratification period. The stratificated seed will have to be sowed right after the end of treatment, avoiding it to dry up.

HS – hot stratification: temperature of the stratification place must be maintained around +/- 20 °C during the period of stratification.

CS - cold stratification: temperature of the stratification place must be maintained around +/- 5°C during the period of stratification.

 3. SCARIFICATION

Process to make the external cover seeds permeable.

Scaldation (recommended): seeds immersion in boiling point water, which is around 82°C, followed by immersion in frozen water, in order to cause a thermic shock. After that, seeds are left in water at ambent temperature water during 24 hours.

Acid scarification (only experts)

Seeds are submerged in sulphuric acid (SO4H2 purity: 90/91% density:1,82) to soften the seed’s hard layer, making the water absorption easier. The immersion time changes according to species and lots. Next, seeds are washed in a jet of water during 5-10 minutes, to remove the rest of acid.

Be careful! The use of sulphuric acid can be very dangerous if personal security conditions are not respected:

-       Avoid skin contact, always use overall, gloves and protection glasses.

-       Prevent water from splashing the receptacle which contains the acid.

-       Make sure that all the tools and receptacles used are made from an acid resistant material.

-       Follow the instructions for use on the product tag.

Mechanical scarification

This method can be use instead of sulphuric acid. Cut with a scalpel or with a cutting tool, part of seed tegument on the area far from ridicule. It can also be carficated by mechanical friction with a rough surface. 

GENERAL RECOMMENDATIONS

Planting season

In general, it is advised to avoid the cold and freezing season, preferring moths from September up to the end of November (autumn sowing) or end of February to end of April (spring sowing). Late sowings can cause secondary lethargies. 

Berry

In a general way, it is recommended to buy seeds without pulp, since most of them contain germination inhibitors. In some species which loose their viability if extracting the seed from the berry it is essential to store them with its pulp, and pulps are to be removed through frictions in water, just before sowing.

Direct sowings

There are species, especially the ornamental ones of hot climate, which are sowed directly. But some thermic conditions of germination have to be respected:

With heat: 20°C by night and 30°C by day

Or with cold: 5°C by night and 15°C by day

Depth of sowing

The depth of sowing is currently 1,5 times the seed size. Nonetheless, in the case of thin and small seeds, it is enough to cover superficially the seed with a thin layer of sand or peat. By this way, we avoid the seed to blow away with wind, to dry out, to be eaten by animals, etc…. 

HS: hot stratification at 20°C

CS: cold stratification at 4°C

(n): number of months

d: duration of treatment

DS: direct sowing

T(): Temperature night-day

BW: Boiling water

SA: sulphuric acid

Sca: scarification

RW: running water at ambient temperature