Sexual (Seed) Propagation
Sexual propagation (also known as seed propagation) occurs when pollen from male anthers fertilizes an egg from a female ovule to produce a seed. The genes from the two parents combine to create a unique third individual plant which inherits certain characteristics of each of the parents but is different from both.
As mentioned in a previous chapter, seeds are composed of three parts: the seed coat, the endosperm, and the embryo. The seed coat is the outer layer of the seed responsible for protecting the seed. The endosperm is the middle layer of the seed and is a food source for the embryo, which is the young plant contained within the seed.
When purchasing seeds, be sure to get them from a reputable dealer. Pick varieties of plants that are adapted to growing in the target area. Select them based on your desired color, size, and growth habit.
Vegetables and flowers can be open-pollenated varieties or hybrids. While the hybrids often cost more, they typically have more desirable characteristics such is more uniformity in the plants produced, more resistance to certain extreme weather and diseases, and production of more crops or flowers than open-pollenated varieties.
Ideally, you should purchase only the amount of seeds you need immediately, though some seeds will keep for a few years if properly stored. Seed packets should contain only seeds for the purchased plant variety, free of weed seeds and other debris. The packets typically include useful information about the seed such as the specific variety, when the seeds were packaged, the percentage of seeds you can expect to germinate, and information regarding any chemicals which might have been used to treat the seeds.
Any seeds purchased well in advance of planting or left over from a previous planting season should be stored in a dry and cool place. Some come in laminated foil packets which should keep the seeds dry. Others may come in paper packets in which case they should be stored in airtight jars or containers in a room with low humidity at about 40° F.
Factors in Germination
Before a seed can germinate, it must meet certain internal criteria like having a mature embryo, containing enough food in the endosperm or cotyledon(s) to last during germination, and having enough hormones to kickstart the process. However, there are four primary external factors that also affect the germination of plants. They include water, light, oxygen, and temperature.
Germination begins when water softens the seed coat so that the seed can begin imbibing water. Once germination has begun, it is important that the medium in which the plant is being grown has a steady supply of water so that the embryo does not die.
Some seeds require light to germinate. Others require darkness to geminate. Yet others have no specific light or darkness requirements to successfully germinate. So read the planting instructions for your variety of plants before sowing. These can often be found on the seed packaging.
When sowing seeds that require light to germinate, it is typically best to simply disperse them on top of the soil. If you desire to cover them, do so with a thin layer of vermiculite or peat moss so that some light will still reach them. If you are planting light-requiring seeds indoors, you can expose the seeds to a fluorescent light source hung 2- to 4-inches above the seeds for 16-18 hours each day.
As discussed in the last chapter, oxygen is necessary for respiration, the process whereby a plant converts stored carbohydrates into energy needed for growth. Germinating plant seeds which respire very actively obtain their oxygen from the air contained in the soil in which they are planted. The oxygen is used in respiration to convert the carbohydrates stored in the endosperm to energy until leaves are formed. For this reason, it is important that you hoe, plough, or otherwise aerate the soil very well prior to planting.
Most seeds have a minimum, maximum, and optimum temperature required for germination. Some have temperature ranges for germination that are very wide while others have ranges that are very narrow. But most plants germinate well between 65° and 75° F. How close to the optimum temperature the planting medium’s temperature (not air temperature) is maintained will determine both how many seeds germinate and how fast they germinate. Maintaining the correct medium temperature to maximize germination percentages is very important and can be achieved by placing germination flats in rooms where the temperature is strictly maintained or on top of heating cables, heating mats, or other devices used to control the temperature of the planting medium.
One of the reasons that some plants go through a dormancy period is to make sure seeds do not germinate until there are favorable environmental conditions. This helps ensure that the new plant has a good chance of surviving. While the seeds of certain plants like annuals do not require a dormancy period, others do. Some trees and shrubs have a difficult time breaking dormancy even when all the environmental conditions are optimal, so they may require special treatments to stimulate germination.
Seed scarification is the process of softening, scratching, breaking, or otherwise scarring the seed coat to promote the imbibition of water so that germination will begin. There are many ways to “scar” a seed coat, including filing the seeds with a metal file, scratching them with sandpaper, or breaking them with a blunt object like a hammer. Certain seeds require hot water scarification, where they are placed into water between 170° and 212° F and soaked for 12-24 hours before they are planted. Other types of seeds may need to undergo warm-moist scarification, where they are placed in an intentionally non-sterile container that is warm and damp so the seed coats can decay for several months before planting.
The seeds of many trees and shrubs will not germinate until they have chilled underground at some low temperature for some minimum period. This is true of most fall ripening fruit and nut trees. Seed stratification is a process often used to artificially induce germination in the seeds of such plants by simulating the required “after-ripening” period of cold weather.
One way to stratify seeds is to plant them about a half inch deep in a small pot or flat filled with sand or vermiculite. Water the plant well and allow to drain. Place the pot or flat containing the moist medium and seed(s) into a plastic bag and seal with a rubber band or twist tie. Place the plastic bag in a refrigerator for 10 to 12 weeks. Refrigerators typically operate in the range of 35° to 45° F which should simulate a winter climate in a temperate region. Check the bag occasionally during those 10 to 12 weeks to ensure the medium remains moist. After 10 to 12 weeks, remove the pot or flat from the refrigerator and place it somewhere warm inside your home. Keep the planting medium moist, and the plants should germinate in a few days. Once the plants are 3 inches tall, transplant them into a larger pot where they can grow until the weather outside is favorable for growth.
Another stratification method involves using sphagnum peat moss. Wet the peat moss, squeeze out any excess water, mix the seed(s) with the moss, and place the moss and seed(s) in a plastic bag. Seal the plastic bag with a rubber band or twist tie and place the bag in the refrigerator. Check the contents of the bag occasionally to ensure the medium remains moist. After 10 or 12 weeks, remove the bag from the refrigerator and very carefully plant the seed(s) in pot where they can germinate. While planting the seeds, avoid breaking the tiny roots and shoots that might have emerged during the stratification period.