Propagating Native Plants

A seed is composed of three basic parts: the embryo, food storage tissue and the seed covering. A mature viable seed contains enough stored food or energy source for seed germination and early seedling growth. The embryo may be immature when the fruit is ripe or the capsule is dry. A period of time in the proper environment is required before germination is possible. The embryo will often increase in size during this period and will develop the properties necessary for germination. Seed coverings can consist of the seed coat and parts of the fruit or seed pod. These structures protect the embryo and food reserve inside the seed. They can also inhibit germination until conditions are suitable for germination and seedling development. The fleshy pulp, the endosperm or the seed coat can all contain chemical inhibitors of germination. Some of these inhibitors are water soluble and can be leached out of the seed, but many can not be washed out and must be broken down by some chemical process. Some seeds have double or multiple dormancy: more than one type of dormancy. An immature embryo may prevent germination of a seed, even though a dormancy due to a hard seed coat has been overcome. A seed may require a cold period to break a dormancy due to the seed coat and then require a warm period for development of an immature embryo. Nandina seeds require a cold-warm-cold sequence of temperatures before germination. Even though some seeds will germinate without a preconditioning period, the germination rate and uniformity might be enhanced by some treatment.

It is often difficult to determine from the appearance of a seed if it is alive and would germinate under the proper conditions. one method of determining seed viability or potential for seed germination is the stain test; Stains can be used to determine if the seed embryo is alive. Living tissue respires in order to maintain its integrity and tetrazolium chloride turns red when it comes into contact with respiring tissue. The seed is cut open and the stain applied to the exposed embryo. If the embryo is alive, it will turn reddish in color. Viable and dead seeds in some plant species can be separated by floating the seed in water. When the seed is filled with the embryo and endosperm to support germination and early seedling growth, the seed will sink in water. When a seed is empty or contains a relatively large amount of air, the seed will float. This is a good means of identifying dead oak and pindo palm seeds, but it does not work with all plants.

Scarification (scaring) and stratification are the two most common means of breaking seed dormancy, hot water soaks and plant growth regulator treatments (chemicals) are also used. Scarification involves the breaking of the impermeable or hard seed coat. This may be accomplished by mechanical means or by acid treatments. Scratching the seed coat with sand paper and cutting the seed coat with a knife are suitable for some seeds. A hammer or pliers can be used to break the seed coat in some cases. Care must be taken in any scarification procedure not to injure the seed embryo. If a specific treatment for a particular plant species is not known, sample seed lots treated with various techniques followed by germination tests is recommended.

Acid scarification has been used successfully in some plants to break the seed coat. Such scarification occurs in nature when the seed coat is partially digested by fungi or bacteria or enzymes in the digestive tract of birds and animals. Sulfuric acid is usually used for scarification. Seeds may be soaked for 5 to 60 minutes, depending upon the toughness of the seed coat and the sensitivity of the embryo. Dry, clean seeds are placed in a nonmetal, noncorrosive, acid-resistant container, and the acid is added slowly in a ratio of one part seed to two parts acid. The mixture should be gently stirred intermittently during the soak to ensure uniform results. Seeds soaked in acid should be washed thoroughly under running water for 10 minutes to dilute the acid in contact or inside the seeds before they are sown. Acids are extremely dangerous and proper handling procedures must be followed. Never pour water into acids, because tremendous heat can be generated in a violent reaction. Dilute acids by pouring the acid into water very slowly. Acid-resistant clothing including gloves, should be worn when handling acids. Acids should be disposed of properly as hazardous material and should never be poured down the sink.

Stratification is the technique of providing a moist chilling treatment to seeds to overcome a dormancy. Stratification may reduce the concentration of germination inhibitors and or increase the concentration of growth-promoting hormones. Seeds should be soaked for 12 to 24 hours and then placed in a moist medium. Coarse vermiculite, sphagnum peat moss, equal volumes of peat and perlite, and coarse sand have proven to be good stratification media that retain moisture yet allow aeration. Seeds should be mixed with one to three times their volume of medium and stored in a container that provides a barrier to moisture loss, such as a polyethylene bag. Seeds can be stratified naturally by placing them outdoors in an area or container protected from rodents. Seeds are usually stratified at 32 to 40 degrees F (0 to 2 degrees C) for a period of 1 to 4 months. Seeds are then separated from the stratification medium and germinated at 70 to 80 degrees F (17 to 22 degrees C). Germination at higher temperatures (>90 degrees F; >27 degrees C) may induce a secondary dormancy.

Soaking in water will often soften seed coats and leach water-soluble inhibitors from the seed to reduce germination time. Best results are obtained when hot water is used, but the temperature sensitivity of the embryo differs with species. Generally, water at 170 to 212 degrees F (67 to 88 degrees C) is poured over seeds in a ratio of one part seed to four or five parts water and allowed to cover the seed and cool for 12 to 24 hours. Changing the water periodically during prolonged soaking is imperative. Seeds should not be allowed to dry after the treatment but should be sown immediately.

Germinating seeds in a controlled environment such as a greenhouse and in a sterile medium will result in optimum seed germination and early seedling development. Problems with weed seeds, nematodes, insects bad weather and diseases are mitigated with such procedures. Plastic, wood or metal flats are common containers for seed germination, although individual containers, celled flats and preformed peat pellets are also used.

The particle size must also be considered in relation to the size of the seed to be sown in the medium. Waterlogged medium cannot supply the oxygen necessary for germination and seedling growth, so care must be taken to provide a good balance between air water and soil. There must be sufficient contact between the seed and the particle for exchange of moisture. A large seed can be germinated in a medium with relatively large particles while a small seed would settle toward the bottom of such a medium. A small seed should be germinated in a medium with relatively small particles to provide an appropriate contact between the seed and the particle.

The proper planting depth differs with seed. Seeds that require light for germination obviously can not be planted deeply or may not be covered at all. Generally, seed should not be planted deeper than two or three times their diameter. Many large seed, especially tropical species like palms, are only inserted into the medium surface where they will remain moist.

Ample water that does not contain a lot of salts must be applied during seed germination and seedling development. Seeds and seedlings differ in their sensitivity to salt levels, but generally the seedlings are less tolerant than mature plants. Optimum temperatures for seed germination and seedling growth vary with plant species . If the specific temperature for a seed species is not known, the range of 75 to 80 degrees F would be a good starting point . Keep in mind that plants native to warmer climates or those that flourish during the warmer months may have higher optimum temperatures than plants that flourish in cooler temperatures. Also reducing night temperatures 5 to 10 degrees F (3 to 5 degrees C) has proven beneficial in germination of some plants native to temperate climates.

Seeds can be grouped according to their requirement of light for germination. Many woody plants do not require light for seed germination, but most epiphytes such as mistletoe and strangler fig require light. Other species have a light requirement for germination, but this can be overcome by chilling or chemical treatments as in lettuce, tobacco and many native seeds. Allium, Amaranthus and Phlox are examples of seeds whose germination is inhibited by light. Some species are sensitive to daylength, but sometimes this can be overcome by temperature treatments.

Emerging roots from a seed need to absorb nutrients. The presence of nutrients in low to moderate concentrations at this time will result in more rapid seedling growth. Application of soluble fertilizers on a periodic basis is recommended.

Seedlings should be transplanted before they over-grow a container and their growth habit or form is altered. Seedlings produced in flats at a high density must obviously be transplanted earlier than seedlings produced in larger containers at a low density. Seedlings must be hardened before transplanting. Hardening refers to a gradual change in the environment so the seedlings can adapt to withstand more stressful conditions than those provided in the propagation phase. The irrigation frequency is usually reduced and the light level and fertilization may be increased. Proper hardening will ensure that the seedlings are established in the production environment at an optimum rate.