Article 4-4 Sexual Propagation
By Cindy Rea
A tiny seed is so complex it holds the complete genetic engineering of an individual plant or being, determining all the characteristics that the new life will possess. Genetic alterations of various plants= seeds have created strains that are designed to grow under specific conditions or produce hybrid species. Horticulturists are able to gain the desirable characteristics of different strains of plants through breeding. Carefully isolating certain strains of plants ensures that cross pollination doesn=t occur keeping the particular strain pure. However most of the world=s vegetation is produced by natural sexual reproduction from seeds.
Sexual propagation is the creation of a seed through the pollination of the female part of the flower by the pollen of the male flower.
Most seeds remain dormant until the introduction of moisture. Once hydration occurs germination proceeds rapidly. The first structure to emerge from the seed is the radicle or embryonic root. The development of the radicle allows the plant to obtain water while creating an anchor for the plant in the growing media. Next the seed divides to permit two leaves to expand and grow. These are the cotyledon or primary leaves. The story of the seed is completed only when the seed has germinated and the seedling has become established.
To propagate a seed successfully it is essential that the conditions be right. Temperature, humidity, light and oxygen and carbon dioxide supply must be favourable to ensure germination. Each kind of seed has specific requirements for germination. Some seeds must be frozen prior to planting, some like a very moist environment while others prefer very dry conditions. For most common seeds the growing medium, in which propagation is to take place, should be sterile and supply plenty of oxygen while retaining moisture.
Rockwool is rapidly becoming the favourite propagation and growing medium for commercial and hobby growers alike. Rockwool is made of rock fiber that is spun like cotton candy and molded into cubes and slabs suitable for different growing stages. It has excellent moisture retention and is very porous allowing oxygen to penetrate freely. One inch germination or starter cubes come in flats of 98 and fit perfectly into a standard nursery tray. The pre-punched hole is the perfect size and depth to receive a seed. Rockwool has a slightly high pH, therefore it should be pre-soaked and pH balanced prior to using.
Soilless mixes are another favourite choice for gardeners. They consist of a pre-sterilized mixture of various media. A blend of peat moss, vermiculite and perlite is most common with some brands adding small amounts of nutrients or possibly a wetting agent. All varieties offer a light, airy mixture with good drainage that seeds will easily germinate in.
Horticultural grade vermiculite is expanded mica that has been sterilized. It is good for starting seeds although it is very light weight and seeds can easily be disrupted.
Perlite, ground volcanic rock, does not have good moisture retention ability therefore it does not make a good propagation medium. It does, however make an ideal covering for the surface of newly planted seeds. It is light weight so new seedlings can easily push their way through the surface towards the light source. It dries out quickly keeping the surface of the growing medium dry which helps to reduce the risk of disease.
Soil possesses many contaminates and it is not recommended for starting seeds unless it is sterilized. Sterilization requires baking soil for at least half an hour in a 200° F. oven. This should kill most harmful organisms. There are also chemical sterilizers available on the market. Soil should only be used for starting seeds if the seedling are to be transplanted into an outdoor garden.
The container used for propagating should also be sterile. Standard plastic nursery trays are inexpensive and reusable. They are available with or without drainage holes and have an optional humidity dome. Flats are ideal for starting a large number of seedlings at once. If you only wish to start a few seeds you can use a plastic tub such as a margarine or yogurt container or a simple styrofoam cup.
Delicate seeds enjoy a warm climate. The propagation medium should be consistently maintained at a temperature of 75° - 85° F. day and night until the first set of true leaves appear. Under normal circumstances, flourescent lamps should provide extra warmth and ideal light conditions for seedlings to thrive. An ordinary incandescent light bulb will provide heat but does not supply the correct light spectrum for plant growth. If you are attempting to germinate seeds in a cooler area such as a basement try to keep the trays off the floor and away from any drafts. A thermostatically controlled heating mat designed specifically to fit under a regular sized nursery tray is a convenient way to make sure seedlings are kept at a comfortable temperature. Once the plants have developed they prefer a cooler temperature of between 65° - 70° F by day and 55° - 60° during the night.
Seedlings develop best with a humidity range of between 40% and 60%. As with temperature, constant humidity promotes healthy even growth. Cover seedling with a clear dome, jar or plastic bag to retain humidity. Allow the seedling to dry out slightly before watering. Gently sprinkle barely warm water on the growing surface taking care not to break fragile stems or wash out seeds. Never soak the growing medium; frequent light sprinklings are preferable. It is best to water as the lights come on as this is the time when most of the growing will occur.
Adequate air circulation and ventilation are necessary to achieve a healthy balance of oxygen (02) and carbon dioxide (C02) in the germination environment. If the air is not replaced regularly the plant will use up all the available C02 close to the leaf and stifle growth. Simply removing the humidity dome for 15 minutes per day and gently blowing on the new seedlings a few times should be enough. As plants mature they can benefit from more C02 than is available in the atmosphere.>
Seeds almost always prefer to germinate in the dark but once the cotyledon sprouts through the surface they immediately need light. Failure to provide light will cause stretching and stem elongation as the new seedling searches for a light source. During the first two weeks seedling should receive 24 hours of light per day. Flourescent lamps are an excellent choice providing a full light spectrum to young seedlings. Metal Halide lamps can also be used but precautions must be taken to keep seedling from burning under the high intensity light. Seedling should be started in the peripheral light and gradually moved directly under the lamp. Once the seedlings are well established gradually reduce the light-on time. Once the true leaves appear past the cotyledon leaves, seedlings should be receiving 18 hours of light per day. The uninterrupted light-off or dark period is essential for respiration and the development of flowers.
After a very short time the young seedlings will have used up all the nutrition that was stored in the seeds. Introduce supplemental nutrition gradually once cotyledon leaves appear and the roots are well established. Start off with a diluted solution of 1/4 strength of the recommendation for mature plants. Gradually increase over a two week period until full strength is reached.
Weak and unhealthy seedlings will quickly fall prey to disease fungus and predators. Over watering, or using water that is too hot or cold will make plants vulnerable. Always start off with fresh seeds and don't overcrowd seedling. Using sterile growing media and trays and maintaining a sanitary growing environment will also help.
If humidity is too high stems may turn brown and become mushy. This may indicate a fungus called damping-off is present. Remove the humidity dome and use a fungicide specifically for seedlings. If leaves start to turn yellow plants may require a little more nutrient. Slowly increase the strength taking care not to over fertilize. Keep a careful eye on the seedlings for any signs of over fertilization. Leaves curling under or the tips burning will indicate that your nutrient is too strong; cut back. Quarantine new plants for at least one week to ensure they are not contaminated with bugs or disease.
Young seedlings should be inspected daily for any signs of trouble or distress. If caught quickly enough most problems can be rectified. The tops and underside of leaves and stems should be checked for irregularities. Pesticides may be used when necessary but should be used sparingly.
Given all the necessary elements, seeds will grow and mature into strong, healthy plants capable of producing flowers and fruit.