An understanding of seed dormancies and how to over come them is essential for the successful germination of some species of seed.
Seed that is viable yet fails to germinate under favorable environmental conditions is said to be dormant. Seed dormancy is a natural occurrence that serves to extend the distribution of species over time. Seed of some species can remain in or on the ground dormant for many years before certain changes take place that will start the germination process. In nature this is a very important and desirable occurrence but in a nursery situation it can be very frustrating to say the least. Therefore to achieve a rapid and even germination it is important to overcome these dormancies prior to sowing seed.
Overcoming dormancy is a relatively simple process for most seed.
The cause of dormancy may be physical or physiological and in some cases double dormancy may exist.
Physical dormancy is usually caused by a hard impervious seed coat which provides a barrier to the entrance of water and gases. Under natural conditions these seeds do not germinate until soil micro-organisms or weathering have sufficiently weakened the seed coat to permit the entrance of water. In a natural situation the time this process takes may vary in individual seeds from a few months to many years.
Physiological dormancies involve growth-regulating systems which may be either due to substances present in the fruit or seed coat that inhibit germination or internal factors within the embryo. In the former, digestion by birds or animals, leaching by rain, degradation by microorganisms or a combination of these break down or remove growth inhibitors from the fruit or seed coat thus setting the stage for germination to begin.
In the latter an after-ripening process must take place within the seed itself. This process usually takes place during a condition known as cold stratification or in some cases warm stratification. Under natural conditions this process happens as the seed lies on the forest floor, where it is exposure to cold and moisture over an extended period of time, thus bring about the internal changes required for germination to proceed.
The purpose of this process is to delay germination until growing conditions are more favorable to survival.
DEALING WITH SEED DORMANCY
Each of our catalogue listings has recommended treatments for dealing with any dormancies that may exist for that species.
Scarification is the process that alters the seed coat to render it permeable to water and gases, used to overcome physical seed coat dormancy. This can be accomplished by a number of techniques, some being more suitable to individual species than others.
1 HOT WATER TREATMENT.
This is by far the most common and one of the easiest ways of scarification. Place a teaspoon of seed in a cup then fill the cup with boiling water, leave to cool and let the seed soak in the water for twenty four hours and then sow. You can usually tell the seed that has been impregnated with water as it will have started to swell. Any seed that is not showing sign of impregnation can be left soaking for a couple of days and if it still hasn’t started to swell then it can be retreated with boiling water.
2 MECHANICAL ACTION
Individual seeds can have there seed coats cut with a sharp knife. Just the out side seed coat is cut to allow the entrance of water. Care should be taken not to damage the inside of the seed or your fingers.
Hold the seed with a pair of flat nose pliers and use a craft knife to cut a slice of the outside seed coat off of the back of the seed. Seeds can also have their seed coats worn down by rubbing on or between layers of sand paper.
After the seeds have been cut of rubbed soak them in cold water for 24 hours before sowing.
3 ACID SCARIFICATION.
Acid treatment involves the digestion of the seed coat with concentrated sulfuric acid. Extreme care must be taken as the acid is very corrosive and reacts violently with water. Wear goggles and protective clothing at all times. Place dry seed in a dry class container and decant the acid over the seeds at a ratio of one part seed to two parts acid. Stir with a glass rod. The aim is to corrode the seed coat until it becomes paper thin. This can be cheeked by taking out samples at intervals and doing a cut test to check the thickness of the remaining seed coat. Once the desired seed coat thickness is achieved the acid is poured off and the seed washed under cold water for 10-15 minutes to remove any remaining acid. The seed can then be planted immediately or dried and stored for later planting.
Familiarize yourself with the correct and safe handling techniques for concentrated acid before you attempt this treatment.
Stratification is a natural process that brings about internal changes within the seed of some species that is necessary before germination will proceed.
It is accomplished artificially by exposing seeds to moisture in cold or warm conditions. There are a number of techniques used to achieve this some being more suitable to individual species than others. The method you chose will depend on the species and the quantity of seed to be treated.
1 COLD STRATIFICATION
This is a simple process of placing seed in cold moist storage. The prerequisites for successful cold stratification are the presence of moisture and air at a temperature above freezing but below 6 deg C. During this cold storage period an after ripening of the dormant embryo takes place thus setting the stage for germination to proceed.
Take care not to let the seed dry out at any stage after stratification has begun and before germination is complete.
When using long stratification periods careful attention should be paid to the condition of the seed in the concluding weeks and if the seed shows signs of germination it should be planted out.
After the stratification period is complete remove from refrigerator sow the seed in trays of seed raising mix place the seed tray in a warm well ventilated spot out of direct sun for germination to proceed (see notes on germination below).
There are several easy ways of achieving this, the method you choose will depend on the quantity and type of seed to be treated and the refrigeration space you have available.
1. Sow the seed in moist seed raising mix in an appropriate seed tray, seal in a plastic bag to retain moisture and refrigerate for recommended period. After the stratification period is complete remove from refrigerator and place the seed tray in a warm well ventilated spot out of direct sun for germination to proceed (see notes on germination below). This method is works well for small lots of seed or if you have plenty of room in the fridge.
2. Spread seed between two layers of very moist blotting paper, seal in a plastic bag to retain moisture (allow some air space) and then refrigerate for recommended period. Turn the bag over every few days to stop the moisture rising to the top of the bag. After the stratification period is complete remove from refrigerator wash the seed off the paper it can then be dry it to dampness to aid handling, sow the damp seed in trays of seed raising mix place the seed tray in a warm well ventilated spot out of direct sun for germination to proceed (see notes on germination below).
3. Mix seed with very moist vermiculite and seal in a plastic bag or airtight container (allow some air space) and keep refrigerated for recommended period. A maximum of 20% seed to 80% vermiculite is recommended. Once stratification period is complete spread the seed / vermiculite mix on top a tray of seed raising mix, place the seed tray in a warm well ventilated spot out of direct sun for germination to proceed (see notes on germination below) .
2 WARM STRATIFICATION
In some cases seed require warm stratification, this process is similar to the above but the seed requires moist storage at temperatures above 15 degrees C to activate the after-ripening process. In most of these cases this is then followed by a period of cold stratification to finish the after-ripening and overcome the dormancy. If this treatment is necessary then it is a simple matter of preparing the seed as discussed above for cold stratification placing in warm storage for the required period then refrigerating for the required cold period.
Germination is the series of events that take place to transform viable seed from its dormant state into a self-sustaining seedling.
The germination of seed that has no dormancy or seed that has had its dormancy requirements satisfied is dependent on several external environmental factors
Seed viability refers to the capacity of the seed to germinate. Potential viability may be ascertained by performing a cut test on a sample of seed. Healthy seed kernels when cut cleanly with a sharp knife should show a firm moist clear or white structure.
The ideal germinating moisture content of the soil varies from species to species. For most seed it is important that the soil is not excessively wet as this prevents aeration and promotes disease, or excessively dry that the germinating seed will wilt. Somewhere in the middle is sufficient to promote germination of most seed.
Seeds require warmth to germinate. The ideal temperature for germination varies from species to species, temperatures between I5 and 25 degrees C will produce good results with most species. Hot pads or soil warming cables may be used to induce germination over winter or in colder areas. When using heating devices it should be noted that an alternating temperature is usually more favorable than a constant temperature to induce germination.
Seeds require oxygen for germination to take place. Most seeds will not germinate when the germination medium is too wet, when the seedsare planted too deep, or when other contitions limit the supply of oxygen.
Although in most seeds light is not required to promote germination, it is required to produce good stocky seedlings and must be supplied during early seedling growth. For this reason and because it does influence germination of some seed, germination should take place in good light, avoid direct sun light as this may over heat or burn the emerging seedlings.
A two stage approach to raising seedlings is recommended. First a quantity of seed is sown and germinated in a seed tray. Then individual seedlings are transplanted into separate containers for growing on to the planting out stage. This method works well for most species and has several advantages over direct sowing in open beds. The main advantage being that one can provide precise environmental control during the critical stages of germination and early seedling development.
Germination trays should be at least 50mm deep and provide good drainage. I prefer trays that are about 300mm by 300mm and usually start enough seed in a tray this size to get about 100 seedlings.
I recommend the use of a commercial seed raising mix for best results, or use a light potting mix with plenty of sand for good drainage.
DISEASES AND PESTS
Damping off is the main problem that may be encountered during the germination process and in early seedling development. It is caused by fungi attacking the seed before or shortly after emergence. The symptoms of damping off are failure of the seedling to emerge, or the death of the seedling shortly after emerging coursed by rotting of the stem at soil level.
The control of damping off disease may be approached in two ways. First it is advisable to avoid the environmental factors that encourage the build-up of fungi in or on the soil, these are warmth (temperatures of 21 to 29 degrees C are the most favorable to damping-of fungi) water logged soil and prolonged damp conditions. They can be reduced by the use of good free draining potting mix, plenty of ventilation. If you still have difficulty with damping off, seed, soil and emerging seedlings may be treated with a suitable fungicide. Several are available from your local garden centre. Leaf eating or sap sucking pests like caterpillar’s aphids etc although not common at this stage, if necessary can be controlled by a pesticide; once again several are available from your local garden centre.
POTTING ON CONTAINERS
Almost any container can be used for growing on seedlings from old cardboard milk cartons to specially designed rootrainers. I personally prefer root trainers which are suitable for small or large scale seedling production. They are designed to provide seedlings with a perfect container environment which promotes optimum root formation. They are easy to fill and hold a minimal amount of potting mix. They are available in a variety of sizes to suit most species of trees and shrubs and can be re-used. The design of the groves and shape of the rootrainers guide roots straight down preventing root spiraling or binding. Roots are then 'air-pruned when they reach the drainage hole at the bottom. Their hinged book design can be opened to inspect root development, soil condition etc at any stage of development without damage to the seedling. This feature also allows easy removal of the seedlings when they are ready for planting out. Because all roots are intact then survival rates are high.
The recipes for potting mixes are many and varied. Most contain differing amounts of some or all of the following, washed coarse river sand, ground bark, decomposed saw dust (not treated) peat, and a mixture of fertilizers and trace elements. The following is one recipe that I have used. One third peat, one third decomposed sawdust or ground bark, one third coarse river sand. Added to this is a slow release npk fertilizer at 3-4 Kg per cubic meter, dolomite at 3-5 kg per cubic meter super phosphate at 1-2 kg per cubic meter and a trace element compound. What ever recipe you use it must be free draining for good aeration yet have good water holding capacity and supply the necessary nutrients for healthy seedling development. If you are not into making your own then most garden centers or landscape suppliers sell ready made potting mix and can usually advise on fertilizer and pH requirements of different seedlings.
PLANTING THE SEEDS
Fill a seed tray with germinating mix, firm down and water well. Sprinkle the seed evenly over the surface. Cover with a fine layer of mix no greater than 1-2 times the diameter of the seed. At this stage trays should be placed in a warm well ventilated shady spot and watered regularly until germination is complete. Avoid over watering as this can create conditions which favor fungal disease. If young seedlings become spindly during early development they may require more light.
When seedlings are 2-5 centimeters high or big enough to handle they should be pricked out and transplanted into individual containers or rootrainers. At this stage it is good practice to cull out any seedlings that don't look strong healthy and vigorous. Pre fill containers with moist potting mix and using a thin stick make a hole in the centre of the container deep enough to accommodate the roots. Very long straggly or distorted roots should be cut off. Lower the seedling into the hole so that the uppermost roots are a few millimeters below the surface, taking care that the roots are not folded or turned upwards firm the soil around them using the stick. To prevent damage the seedlings should be handled with care by their outer leaves not there stems or growing tips. Care should be taken not to allow there roots to dry out. After transplanting, seedlings should be watered and placed in a shaded area to be hardened off before they are moved to were they are to be grown on to the planting out stage.