Plant Propagation (Part 1: Sexual Propagation)


“Plant propagation is the process of multiplying the numbers of a plant.”  It is a natural process that has been proceeding for millennia. Man has simply added a few new twists. There are two types of plant propagation sexual and asexual.

Sexual reproduction is via seed. This involves the combining of a male gamete and a female gamete so that the offspring will have characteristics of both parents. This is how genetic diversity is maintained. In plants it is generally called hybridization.

Asexual reproduction involves the “multiplying of plants from stems, roots, leaves or other plant parts without the union of male and female gametes.” All offspring will be identical genetically to the parent plant. Cloning is a form of asexual production at the cellular level. Rooting a cutting is asexual reproduction on a larger scale.

Sexual reproduction is based on the process of pollination. In this procedure the male pollen is deposited on the stigma of the flower. A pollen tube grows and carries the pollen from the stigma to the ovary where the egg is fertilized.

Pollination can be of two types. In open pollination the combination of egg and sperm is random. A bee comes along and deposits pollen from one tomato flower onto another. It doesn’t make a choice but  simply randomly spreads pollen among the tomato plants. The resulting seed is a chance combination of genes. Open pollinated plants can hybridize by a natural process, although many flowers are so constructed so that they self- pollinate without the helpful bee. Self-pollinating plants produce a fairly stable seed that has the characteristics of the parent plant.

Hybridization of the manmade variety is the deliberate controlling of the genetic makeup of a plant by the cross pollination of genetically different parents. Most plants are now hybridized. How is this done? First the parental lines must be stabilized. This is accomplished by inbreeding a line until the characteristics of each plant are the same from every seed that is produced. Once the lines are stable, a time consuming process, they are then cross bred. The crosses will produce a new type of plant but it will be a standardized plant from each seed. This is why the hybrid seed that come from a catalogue all produce the same plant while randomly hybridized seed from your garden will not breed true.  Hybrid plants are often more vigorous as a result of genetic recombination and, of course, when bred by man they are selected for desirable traits.

When buying seed buy only enough for the current year and check the date on the seed packet. Fresh seed is essential to good germination. While normal seeds can be dried and stored from anywhere from 3-100 years and remain viable, there are seeds that cannot be stored dry. These seeds, when dried to less than 20% moisture content, will not germinate. They must be germinated immediately after harvesting Some seeds are treated in a process that is the equivalent of pre-germination. These seeds germinate more quickly and uniformly but cannot be stored successfully.

If seeds are to be stored, either from purchased sources or from the garden, they should be stored in a cool dry place. The seeds should be placed in packets in a tightly closed container. The relative humidity and temperature should not add up to more than 100. If seeds are stored in the refrigerator at 40⁰F the humidity should be under 60%. If they are stored at 50⁰F the humidity should be under 50%. Under moist conditions pathogens become active causing the seeds to rot. If the storage area is too hot the metabolic processes of the seed speed up. A desiccant can be used in the seed container to keep the humidity down.

Before planting stored seed a germination test can be performed to check viability.  All that is needed is a plastic bag and a damp paper towel. A number of seeds are placed on the towel; ten makes the math easier although so few are not going to give a scientific sampling. It is however accurate enough for home use. The towel is placed into the bag and sealed. The package is then placed in a situation that is conducive to the germination of the type of seed used. The germination is checked daily. The counts will indicate what percentage of the seeds is viable and over what period of time they will germinate. If there is only a 50% germination rate on a stored batch of seeds a greater number of seeds will have to be sown to get the desired number of plants than if the germination rate is closer to 100%. Seeds that are collected from native wildflowers will often have a low germination rate and will germinate over a longer period of time.

For a seed to germinate four factors are required: water, light, oxygen and heat. The first step of seed germination is the rapid uptake of water. The water hydrates the seed and activates enzymes that set growth in motion. The radicle or seed root is the first thing to emerge from the seed. This is followed by the hypocotyl (seed stem) and cotyledons or seed leaves. It is crucial that seeds do not dry out in this phase.

Some seeds require light to germinate. These are generally small seeds that will germinate close to the top of the soil and have a limited supply of stored food. Some seeds require darkness to germinate. Most commercial seeds, however, have the light/dark requirement for germination bred out of them. Unless there is a light requirement on the package or the seed has come from a wild source this will probably not be a factor. There are resources on-line where the full germination requirements for specific seeds can be checked. (  Perennials are more likely to have specific germination requirements than annuals.  Light become critical when the cotyledons emerge. Without sufficient light the seedlings will etiolate or grow tall and spindly.

Oxygen is required by the growing seed to carry nutrients across cell walls. While growing plants produce oxygen and use carbon dioxide during photosynthesis, germinating seeds need an adequate oxygen supply. A well aerated growth medium is necessary so that adequate oxygen is available to the roots.

Each type of seed has a temperature range over which it will germinate most effectively. For most seeds this range is from 70-75⁰ F. For seeds that originate in tropical climates, and this includes most houseplants, the optimal temperature is closer to 80⁰ F. Cool weather plants like pansies germinate best at a temperature range of 55-65⁰ F.  Special heating pads are available to set up ideal temperature controls for seeds. These apply bottom heat and maintain the soil at a given temperature. Soil is usually cooler than room temperature due to evaporation.

Seeds, unless they are in the process of germination, are in a dormant condition. Just adding water will break the dormancy of most seeds. There are seeds, however, that are more difficult to set on the road to germination.

There are two types of dormancy that require special treatment. One is physical dormancy where the seed is coat is so hard or is covered by a scarwaxy coat so that water cannon penetrate. The seed coat must be treated so water can enter the seed. There are a number of ways to do this. Mechanical seed scarification can be done by nicking or removing some of the seed coat by using a file or sandpaper. In hot water scarification the water is brought to near boiling and the seed is soaked for 15-20 minutes. Commercially, seeds are often soaked in acid for 5-30 minutes.

In chemical dormancy the seed contains some sort of inhibiting substance that must be broken down before the seed will germinate. This is commonly done by exposing seeds to dark, moist and cold conditions for a specific period of time. This process mimics a winter season and breaks down the chemical compounds that prevent seeds from germinating prematurely in temperate regions.

This may be done in a refrigerator by placing seeds in moist sand or peat moss, sealing them in plastic and keeping them in the cold for from 4-8 weeks. Different seeds will require different lengths of time. Placing the seeds in a container and burying the container outdoors for the winter is an alternate method. Soaking the seeds in 1000 parts of gibberellic acid for 4-16 hours will bypass chemical dormancy in some seeds.

The most important thing in starting seeds is probably the growing media.  It should be fine and uniform; well aerated and loose; does not crust; holds moisture; is free of insects, disease and weed seeds; is low in fertility and has a pH of 6.0 to 6.8.  Commercial growers frequently use seed plugs to avoid transplant shock. Seed starting mixtures can be purchased premixed and they should meet the above criteria. It is suggested that they should be pretested before committing large batches of seeds to any particular brand.

It is possible to mix your own seed starting medium. The proper ratio is one part sterilized soil; one part sand, vermiculite or perlite and one part peat moss. Soil may be sterilized in the oven by heating moist soil to 180⁰ F and holding it at that temperature for 30 minutes. It can also be done by placing the soil in the hot sun in a black plastic bag until it heats up.  This won’t kill all organisms but it will kill plant pathogens. A sterile soil in which everything is dead will encourage external microorganisms to move in. Often this takes the form of a brownish grey fungus across the top of the soil.

The medium should be wet but not soaking before the seeds are planted. Since peat moss it difficult to wet, warm water is more effective than cold. A drop or two of soap added to the water will act as a surfactant and make it easier to wet the peat moss. No perfumed or antibacterial soaps should be used.

Almost anything can be used as a container to germinate seeds but the easiest to use are closed  plastic containers such as those that are used to conthouse deli foods. They should be transparent and with a lid. The alternative is to start seeds in containers that will fit into plastic bags. This makes keeping them from drying out much easier. All containers should be washed in a 10% bleach solution.

When seeds should be started is dependent on a number of factors. The easiest way is to add the germination time to the days it takes to bring the plant to transplant size and count back from the last frost date for your area. If plants can go out before the last frost date, such as cabbage or parsley, this should be taken into consideration.

Germinating seeds should be kept moist but not overly wet. Containers should have adequate drainage. A good quality of water is essential, as high salt levels can prevent growth and germination. Use tepid water and be careful not to wash the seeds out.

Keep soil temperatures at around 70-75⁰ F unless otherwise specified for the seed. Give the plants as much light as possible after they germinate. If adequate daylight isn’t available place florescent or LED lights as close to the plants as possible.  Begin to fertilize the seedlings as soon as the first set of true leaves emerge. Start with about ¼ of the recommended strength and move up to ½ as the seedlings get larger. When transplanting handle the seedling by the leaves , as damaging the stem will kill the small plants.

There are a number of problems that commonly occur when growing plants from seed. Weak, leggy plants are the result of not enough light, too much water, conditions that are too warm, excessive fertilizer or too high salt levels, or plants that are too crowded. Damping off, a condition in which the plants collapse at the soil level, is caused by 3 different pathogens. These are encouraged by excessive soil moisture, low temperatures before germination, high temperatures after germination or over watered flats and restricted air movement. Dwarf plants are caused by low temperatures or over sterilization of the soil, which can change manganese to a toxin. If the plants are small and turning yellow insufficient fertilizer may be the culprit. Excess fertilizer will turn the plants dark green and then they will start to yellow.  Poor root growth can be caused by poor soil aeration, poor drainage, fungus gnats, lack of fertility, excess of soluble salts or low temperatures in the root zones.

This concludes the session on sexual propagation in plants. The next section explores asexual propagation, which include division, rooting, grafting and budding.