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PLANT PROPAGATION | |
Most
plants reproduce more of their kind through production of seeds. This is
SEXUAL REPRODUCTION and it involves the exchange of genetic material
between two parent plants. Many ornamental plants do not come "true" from
seed. To increase the numbers of these plants, gardeners and
horticulturists use ASEXUAL PROPAGATION. In asexual propagation, the new
plants are genetically exact copies or clones of a single parent plant.
The methods used in asexual propagation range from taking leaf cuttings of
African violets to grafting apple cuttings onto root
stocks.
KEY CONCEPTS
I. Sexual Propagation of Plants
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- Collecting
- Germinating
II.
III.
SEXUAL PROPAGATION OF PLANTS
Sexual propagation of plants involves the exchange of genetic material between parents to produce a new generation. Sexual propagation offers the following advantages:
- It is usually the only method of producing new varieties or cultivars.
- It is often the cheapest and easiest method of producing large numbers of plants.
- It can be a way to avoid certain diseases.
- It may be the only way to propagate some species.
Propagation by seed is the most commonly known method of producing new plants. Common annual and biennial vegetables and flowers are grown easily from seed. Perennials grown from seed may take more than one season to flower. Lawn grasses are commonly grown from seed by the home gardener.
Use seed that is guaranteed reliable for freshness and purity. Purchase seed that is packaged for the current year. Most seed companies provide information on expected percent of germination. This figure tells what percent of seed is expected to sprout or germinate. Generally, 65-80% of most fresh seed sown will germinate. Of those seeds that germinate, 60-75% will grow into satisfactory seedlings.
When choosing seed, be sure to select varieties or cultivars that meet your requirements for size, color and growth habit. Choose varieties that will mature before frost. This can be estimated by using the number of days to maturity (found on the seed packet or in the catalog) and comparing it to the days between probable planting date and the average frost date of the area.
When considering seed not packaged for the current year, remember that the germination rate of most seeds drops dramatically after the first year.
Seed catalogs are not only helpful in purchasing seed, but they are also excellent references for cultural information. Catalogs and seed packets may provide information on country of origin, bloom time, specific germination requirements, cultural requirements and disease resistance. Some seed packets indicate if seeds have been chemically treated to prevent disease. Read packets and catalogs thoroughly and follow their recommendations. The more information the gardener possesses, the greater the chance of growing plants successfully.
Seed saved by the home gardener will probably be the result of random pollination by insects or other natural mechanisms. Random pollination results in seeds that produce plants that may not be identical to the parent plant. The seeds of HYBRID cultivars should not be saved.
Some plants make excellent candidates for seed saving. Common self-pollinated, non-hybrid and purebred annual vegetable seeds that can be saved include lettuce, beans, peas, herbs and HEIRLOOM tomatoes.
Saving seed saves money. It allows the gardener to maintain varieties that are not sold commercially. Many avid seed savers belong to groups that exchange seed through networks. Some seed saver groups specialize in keeping heirloom varieties. Many heirloom varieties are the great-grandparent plants of modern cultivars.
It may be tempting to bring home seeds or plants seen on vacation in foreign countries. However, this is how many serious insect and disease pests are introduced. A nonnative plant may become a noxious weed. Follow all import regulations for horticultural materials. The USDA Animal and Plant Health Inspection Service can provide information to travelers.
It is important to save seed from healthy plants because some diseases can be carried in seeds. Commercially grown seed is protected from disease problems because it is produced under very strict conditions with frequent inspection.
Harvest seed just before fruit is fully ripe. For flowers with exposed seeds, place the seed stalk or flower head in a bag and store in a warm, dry location. Seed will fall into the bag when it is completely dry. The seed of pulpy fruits should be separated from the pulp, washed and thoroughly dried.
Once seeds are completely dry, place them in airtight storage containers marked with name and date saved. Store seeds at 40 degrees F with low humidity. The refrigerator provides these conditions.
Seed of many plants can remain VIABLE for up to 5 years if properly stored. However, it is best to use home-harvested seed during the following growing season. Some species of plants produce seeds that are short-lived. These seeds must germinate immediately after they ripen or they lose their viability. Delphinium, onion and parsley are examples.
Before planting, it is a good idea to check stored seed for its germination rate. Planting these seeds directly in the garden may be a waste of time and effort if germination rate is very low. To check germination rate, place some of the seeds between paper towels that are kept constantly moist and between 65 and 70 degrees F. Check the seeds daily for germination. If the germination rate is 70% or less, consider buying new seed.
When germinating seed it is helpful to remember that a seed is made up of three parts:
- an outer protective coat
- a food supply under the seed coat (the endosperm)
- an embryo of a young plant
The protective coat prevents sprouting until ideal growing conditions exist. Bringing seeds out of dormancy involves manipulating conditions to hasten germination. Even with ideal conditions, some seeds are still very difficult to germinate.
There are several factors that affect germination. Water (moisture), light (or dark), oxygen and heat play a part in triggering germination. In addition to environmental factors, seed must be viable.
Water is essential in the first phase of germination. Water penetrates the seed coat and causes the endosperm to swell. The seed coat, softened by water, splits open as the endosperm swells. The water dissolves nutrients in the endosperm making them available to the embryo and growth begins.
The growing medium must be constantly moist, but not wet. Any dry period may cause death of the sprouting embryo.
Light can stimulate or inhibit a seed's germination. This determines whether the seed should be sown on the surface of the growing medium or below the surface. Check the seed packet or catalog for light requirements.
Oxygen is required by the embryo to begin growing. The seed must respire to break down the food stored in the seed. This is one reason for using a light, well-aerated growing medium for starting seeds.
Every seed has an optimum temperature range for germination. Many seeds have a fairly wide temperature range for germination, but some are limited to a narrow range. The temperature range is usually given on the seed packet or in the catalog.
The temperatures required by many seeds are higher than those in most homes. The desired constant temperature can be achieved through heating cables placed under germination containers. Setting flats or pots on radiators, the furnace or on the refrigerator will provide bottom heat. However, these locations may be too hot and cause the soil to dry too quickly.
Once germination occurs, a different, usually lower, temperature may be required for optimal growth of the seedlings.
The coat of certain seed is extremely tough and must be penetrated by special means. Particularly hard seed may be scarified. Scarification involves breaking, scratching or softening the seed coat to allow moisture penetration.
Two methods of scarification commonly used by the home gardener are mechanical and hot water. Mechanical scarification involves breaking or weakening the seed coat with a file, sandpaper or hammer. Hot water scarification involves placing seeds in water that is 170 to 210 degrees F. After the water cools, seeds should continue to soak for 12 to 24 hours. Then they are planted. Specific instructions for scarification are usually mentioned on the seed packet or in the seed catalog.
Some seeds will not break their dormancy unless exposed to a period of low temperature and moist conditions. Stratification requirements are usually indicated by the seed supplier. They can also be found in references, such as Michael Dirr's MANUAL OF WOODY LANDSCAPE PLANTS, Steven Still's MANUAL OF HERBACEOUS ORNAMENTAL PLANTS and plant propagation texts.
Plants that typically require seed stratification include many trees and shrubs, and certain perennials.
This period of "chilling" or stratification can be accomplished by placing seed in a moist, sterile (pasteurized) growing medium, such as a mix of equal parts clean sand and peat or sphagnum peat moss, in a disinfested container. Enclose the container in a tightly sealed plastic bag and place it in the non-freezer section of the refrigerator. Towards the end of the stratification period seeds may start to sprout. Handle sprouted seeds carefully when transplanting into pots.
Some seeds may require both scarification and stratification to germinate reliably.
Sowing seed indoors is the easiest and cheapest method of producing vegetables, annual flowers and some perennial plants. Plants with extremely small seeds or those that need a long growing season make excellent candidates for starting indoors.
Very small seed is difficult to plant outdoors because of size. Plants that require a long growing season may not have enough time to reach maturity unless started indoors in winter or early spring.
Supplies needed for indoor seed sowing include the following: fluorescent or grow lights, disinfested containers with excellent drainage, pasteurized (sterile) seed-starting medium and a location with proper temperature and ventilation.
Choose a medium with a loose, uniform, fine texture. A pasteurized mixture that is 1/3 soil, 1/3 sand, vermiculite or perlite, and 1/3 peat moss has the qualities of a good seed-starting medium. Retail garden centers carry mixes labeled for seed starting. Whatever is selected, be sure it is pasteurized (sterile). Using pasteurized soil prevents damping-off, a fungal disease that kills young seedlings. Pasteurized soil also helps to avoid weeds, diseases and pests.
Seed-starting media are usually low in fertility. This means that a regular fertilization program is very important once seedlings emerge.
Any recycled containers are adequate for seed starting provided they are disinfested, have good drainage and are at least 2 inches deep. Other container options include compressed peat pellets, peat pots, paper pots, plastic cell packs and flats.
Peat and clay containers tend to dry more quickly than plastic containers because they are very porous.
The correct timing of seed sowing is an important factor in successful indoor seed starting. In winter months, overanxious gardeners may sow seeds too soon. Seedlings that are held indoors too long perform poorly once transplanted into the garden. Most seeds should be sown 4 to 12 weeks prior to transplanting into the garden. The time it takes for seedlings to be ready for transplanting outdoors will vary.
An ACCLIMATION period before placing seedlings directly into the permanent growing site must be included. However, readiness for outdoor planting will vary with how quickly germination occurs, the growth rate and weather conditions. Quality and quantity of light, temperature and nutrients affect the growth rate. Seed catalogs and packets provide information on days to germination and weeks needed to reach transplant size.
Fill the container to within 1/4 inch of the top of the container with moistened seed-starting medium. After sowing seeds, keep the medium moist, not wet. To keep the medium moist, you may place the container in a plastic bag just large enough for the container. Seal the bag. The plastic bag keeps moisture in, but allows air exchange. The plastic bag method should not require any further watering until germination. Provide proper light and temperature conditions.
Once seedlings germinate, remove the container from the plastic bag. Place the container in a location that has high light intensity and cooler temperatures.
Use the following rules to sow seed, depending upon seed size. Sow very small seeds by sprinkling on top of the medium and pressing in. Use a fine mist of water to gently wash seed into the growing medium. Sow medium-size and larger seed in rows 1 to 2 inches apart, and 1/8 to 1/4 inches deep. If no depth is specified on the seed packet, use the general rule of planting the seed at a depth twice the diameter of the seed.
When sowing in a tray or flat, sowing in rows is preferred over scattering seed. This method provides better air circulation than scattering of seed. When seedlings are crowded, they may become tall and spindly. To avoid the need to transplant seedlings from a seed flat to pots, you may sow seeds directly into cell packs or peat pots. Plant two or three seeds per cell or pot. When they germinate, remove the two less vigorous seedlings.
CARE OF SEEDLINGS STARTED INDOORS
TRANSPLANTING
Once the seeds have germinated, you will see two seed leaves or cotyledons. Eventually, these seed leaves will wither. Next, true leaves will form. These true leave look like the plant's typical leaves. Transplant the seedlings to individual containers once the first set of true leaves appear.
Use a pencil, small stick or other narrow tool to lift seedlings from the seed flat. Plant each seedling in its own small pot filled with pasteurized growing medium. The new container should be no larger than a 2-1/2- to 3-inch pot.
Hold the seedlings by a leaf between the thumb and forefinger. Handling by the leaf avoids damaging the fragile stem which would kill the seedling. If a small leaf is lost in transplanting, the plant can recover and grow normally. Expose seedlings' roots to air as briefly as possible. Lightly firm the soil around the seedling.
Transplanting temporarily slows or stops the growth of seedlings. Sowing into cell packs bypasses the need to transplant the seedlings.
WATERING SEEDLINGS INDOORS
Watering can be a cause of seedling failure. Keep soil moist but not wet. Small, tender seedlings dry out rapidly and can die. Remember that roots always must have oxygen, as well as water; therefore, do not keep the soil soaked. Water when the surface of the soil begins to dry out.
Bottom watering helps prevent damage to the seedlings caused by a hard stream of water. Bottom watering also encourages deep root development and ensures that the entire depth of soil receives moisture. Do not let the pot or flat sit in water longer than it takes for all of the soil to become moist.
TEMPERATURE FOR SEEDLINGS INDOORS
Keep seedlings in a well-ventilated, cool location. The temperatures should be 55 to 60 degrees F at night and 65 to 70 degrees F during the day. These temperatures encourage compact, bushy, vigorous growth while minimizing disease.
LIGHT REQUIREMENTS FOR INDOOR SEEDLINGS
Seedlings require bright light immediately after germination. One warm-white, 40-watt bulb and one cool-white, 40-watt bulb used together are adequate for seed starting and seedling growth. Fluorescent lights can be used for one year before replacement is recommended.
Special grow lights are also suitable, but more expensive. The lights should be no more than 6 inches above the top of the seedlings. Mount the light fixture so it can be raised as the plants grow in height.
Day-length requirements vary with different plants. Petunia, snapdragon, phlox and China aster require short daylight periods of 10 to 12 hours. However, most plants that are started from seed benefit from 16 to 18 hours of light.
FERTILIZING SEEDLINGS INDOORS
Because the growing medium used to start seedlings is usually low in nutrients, a regular fertilization program is important for proper plant growth. Apply a liquid fertilizer high in phosphorous weekly. Fertilizer with a 1-2-1, N-P-K ratio is recommended. Dilute fertilizer 1/4 to 1/2 the label's recommended strength and apply sparingly. Always use a liquid form of fertilizer.
PINCHING SEEDLINGS
Pinching the growing tips of seedlings will result in more branching. This produces a fuller, stockier plant.
HARDENING OFF SEEDLINGS BEFORE TRANSPLANTING INTO THE GARDEN
Plants grown indoors must be gradually introduced to outdoor conditions. If seedlings are not hardened off, leaves may be burned by the intensity of the sun or damaged by wind. Acclimate plants by first placing them in a cool, protected location, such as a porch or shaded COLD FRAME.
This first step in hardening off allows plants to adjust to outdoor temperatures. After 7 to 10 days, move seedlings into a shaded area of the garden for 2 to 3 days. This will prevent sunscald. Finally, hardened seedlings can be planted directly into the garden as weather permits. Planting on a cloudy day or late in the evening is a sensible precaution.
TRANSPLANTING SEEDLINGS INTO THE GARDEN
The garden soil should be adequately dry to prevent compaction. At this point, plants will again experience transplant shock and a setback in growth. Plants must adjust to dramatically different nutrient levels, soil temperatures, moisture levels and soil tilth in the garden.
Pull apart the lower portion of the root mass to get the roots growing outward. If seedlings have been grown in peat pots, pull apart the bottom of the pot and roots. Although seedlings may be planted without removing the pot, be sure to maintain the same soil level. Trim away any of the pot that is above the soil line. The exposed portion of the peat pot acts as a wick and dries out the entire pot and roots.
Water seedlings into the soil. A cup of transplanting solution will help plants get off to a good start. Make your own transplanting solution by mixing 1 tablespoons of a water-soluble 20-20-20 fertilizer in a gallon of water.
SOWING SEED DIRECTLY INTO THE GARDEN
Many flowers and vegetables may be sown directly into the garden. Direct sowing avoids transplant shock. It takes less work but involves more risk from weather, pests, diseases and erosion.
Before sowing seeds directly into the garden, know what conditions are required for germination and growth. A warm-season crop, such as beans, may rot before germinating if planted in cold, damp soil. Knowing the average frost date for your area helps to avoid losing frost-sensitive plants. Some cool- season crops, such as peas and lettuce, should be planted early in the season while temperatures are low.
Sow seeds in a row or broadcast them into a well-raked seedbed. The seedbed should be free of stones or other large debris. Choose a calm day. To broadcast seeds, merely scatter them over a large area in the seedbed. Cover the seeds with a fine layer of soil. To sow very small seeds, mix them with sand before scattering. Then water with a gentle spray. Avoid washing seed away when watering.
Ferns can be propagated from SPORES which develop in clusters on the underside of FRONDS. Collect fronds that have produced spores and store them in an envelope until dry. After drying, separate the dust-like spores from the cases by screening. Store spores in an airtight container in a cool, dry place until ready to plant.
Germinating spores requires more time and care than germinating seeds. Growing ferns from spores involves the two different generations of ferns. Spores first produce an asexual plant called a GAMETOPHYTE (gam- EAT-oh-fight). This plant is very small and has none of the usual plant parts. It resembles a moss-like growth and is about 1/8 inch thick.
The gametophyte reproduces sexually and forms SPOROPHYTES (SPORE-oh-fights) which have visible roots, stems and leaves.
During the first phase of growing ferns, sterile conditions are critical. Moss, fungi and algae compete aggressively with young fern gametophytes. Sow spores on top of a pasteurized (sterile), moist, soilless mix or sphagnum peat in a disinfested container. Water must also be sterile. It takes from 3 to 6 months to grow ferns from spores.