Detailed Explanation of Key Technical Points for Flower Seedling Cultivation in Greenhouses

Flower propagation is the means by which flowers reproduce offspring and maintain germplasm resources. Only by preserving germplasm resources and propagating them to a certain quantity can they be used for landscaping and provide conditions for flower selection and breeding. There are many species and cultivars of flowers, and propagation methods vary greatly. They can be divided into two methods: sexual propagation and asexual propagation.

Sexual propagation: also called seed propagation, refers to propagation using seeds, such as sowing.

Asexual propagation: also called vegetative propagation, refers to the method of using the nutritional organs of flowers (roots, stems, leaves) for propagation to obtain new plants. It usually includes division, cuttings, grafting, layering, and tissue culture.

I. Sowing method: Sowing of greenhouse flowers is usually carried out in the greenhouse, and is less affected by seasons. The sowing time is determined according to the desired flowering period.

Before sowing, seeds can be appropriately treated according to their specific conditions. For example, those with thicker seed coats can be soaked in warm water or sulfuric acid, and some seeds can be stratified in sand.

The sowing soil can be a mixture of leaf mold, sandy soil, and garden soil. The ratio depends on the size of the seeds: fine seeds are mixed at a ratio of 5:3:2; medium-sized seeds at 4:2:4; large seeds at 5:1:4.

Sowing can be done in a hotbed, or in containers such as pots, sowing trays, or plug trays. Except for plug trays, which can use sowing soil directly, hotbeds, pots, and sowing trays should be arranged from bottom to top as small stones or other coarse particulate material — coarse culture soil — sowing soil, each accounting for 1/3.

Fine seeds are suitable for broadcast sowing, and can be mixed with fine sand for broadcasting or broadcast alone. Sowing should not be too dense. Medium and large seeds are suitable for dibbling or drilling. Greenhouse flowers suitable for broadcast sowing include: Begoniaceae such as wax begonia, tuberous begonia, Rieger begonia, etc.; Convolvulaceae such as petunia, etc.; calceolaria, dianthus, trailing lobelia, and campanula.

For fine seeds sown by broadcasting, after sowing, a very thin layer of fine sandy soil can be applied, or no soil cover is needed, but after watering or soaking the pots, a layer of plastic film or glass must be placed over the hotbed and containers to increase humidity and prevent the seeds from drying out. For medium and large seeds, the covering depth after sowing depends on seed size. The covering depth for large seeds is 2-3 times the seed thickness, while for small and medium seeds it is generally until the seeds are no longer visible. After sowing, attention should be paid to the humidity of the hotbed and containers, and water should be sprayed regularly. After seedlings emerge, the film and other covers should be removed. For broadcast-sown seeds, when the seedlings grow to about 2 cm tall, thinning should be carried out in time.

II. Cutting method

The propagation method of using the nutritional organs of plants (roots, stems, leaves) for cuttings to form independent plants is called the cutting method. Plants cultivated by this method grow faster than seedlings grown from seed and can be cultivated into large seedlings in a short time, especially for some plants that are difficult to flower. For ornamental flowering plants, it can bring flowering earlier; however, cutting-propagated plants have no taproot, and their root systems are weaker than those of seed-grown seedlings.

(1) Cutting substrate: There are many substrates used for cuttings, mainly soil, sand, perlite, vermiculite, and water. Water cuttings are a relatively hygienic and simple method, but attention should be paid to changing the water frequently to keep the water clean. Cooled boiled water is preferable. If tap water is used, it should be left to stand for 2-3 days. To prevent bacterial growth, steel coins, charcoal, etc. can be added to the water. Flowers suitable for water cuttings include croton, Araceae (lucky bamboo, split-leaf philodendron, pothos, etc.), greenhouse impatiens, Begoniaceae (wax begonia, silver-spotted begonia, etc.), Ficus (rubber tree, banyan, etc.), peperomia, lucky bamboo, clearweed, umbrella plant, spiderwort, clematis, etc.

(2) Types and methods of cuttings

According to the different nutritional organs selected, they can be divided into three types: leaf cuttings, stem cuttings, and root cuttings.

1. Leaf cuttings

Mainly refers to using plant leaves as cuttings to make them root and grow leaves, thereby becoming a complete plant. Generally, these leaves have thick petioles, veins, or fleshy blades. The leaves selected must be fully grown.

(1) Whole-leaf cuttings: Using a complete leaf as the cutting. The leaf can be placed flat on the substrate, but it must be ensured that the leaf is pressed tightly against the substrate, so iron nails, bamboo skewers, or the substrate can be used to fix the leaf, such as plants in Crassulaceae (mother of thousands, etc.) and Begoniaceae (begonia, rex begonia, etc.). The petiole can also be inserted into the substrate while the leaf lies flat or stands upright on the substrate. This method is suitable for plants that can produce adventitious buds from the base of the petiole, such as gloxinia, African violet, peperomia, etc.

(2) Leaf-section cuttings: Cutting a complete leaf into several pieces and rooting each separately. Each leaf piece can form adventitious buds. Examples include snake plant, gloxinia, pepperomia, and plants in Begoniaceae.

2. Stem cuttings: Using stem segments as cuttings for propagation. According to different cutting seasons, they can be divided into softwood cuttings and hardwood cuttings. Softwood cuttings are carried out during the growing season, and the cuttings are shoots that have not fully lignified. Hardwood cuttings are carried out during the dormant period or at the beginning and end of the growing season, and use fully lignified shoots as cuttings.

3. Root cuttings: Some flowers can produce adventitious buds from roots, thereby forming new plants, so such roots can be used as cuttings for propagation. Roots used for cuttings are generally relatively thick, and some are even slightly fleshy, such as garden phlox, Chinese peony, etc.

(3) Methods to promote rooting of cuttings: In order to make cuttings root as early as possible, some methods can be adopted, such as chemical treatment. Hormone treatment is a very common means. Commonly used hormones include indolebutyric acid, indoleacetic acid, naphthaleneacetic acid, and rooting powder. In addition, some chemical substances such as potassium permanganate and sucrose can be used. Potassium permanganate is more effective for most woody plants, generally at a concentration of 0.1-1.0%, with a soaking time of 24 hours. Sucrose is effective for both woody and herbaceous plants, with a treatment concentration of 2-10%, generally soaked for 24 hours. There are also many physical treatment methods, commonly including ringing, misting, hot water treatment, increasing bottom heat, and bottom-heat treatment.

III. Division method

This method is one of the methods of vegetative propagation of plants. It refers to the propagation method of separating small young plants growing on the plant body, or separating part of the nutritional organs of the plant body from the mother plant, and planting them separately to form independent plants. It mainly includes the division method and the use of plant nutritional organs such as suckers, bulbils, and modified stems (including runners, creeping stems, climbing stems, rhizomes, corms, bulbs, and tubers).

1. Division: Cutting off the tillers at the rhizome or underground stem and planting them to form new plants. Examples include cymbidium orchid, hosta, etc.

2. Suckers: Shortened, thickened, rosette-shaped short branches that naturally arise at the root zone or in the leaf axils of aboveground stems of certain plants. Examples include aloe and bromeliads.

3. Bulbils and aerial tubers: These are special forms of buds possessed by certain plants, born in leaf axils or inflorescences. Some flowers in Liliaceae all have them, such as lily, tiger lily, ornamental onion, etc. After bulbils and aerial tubers separate from the mother plant, they can naturally fall to the ground and root.

4. Modified stems: Refers to certain hereditary changes that occur in plant stems during evolution, including aspects of morphology, structure, and physiological function. Among them, modified stems that can be used for propagation mainly include runners, rhizomes, bulbs, corms, tubers, etc.

Runners are modifications of aboveground stems. They are stems with relatively long internodes that emerge from leaf clusters, and they bear leaves, flowers, and adventitious roots at the nodes. They can also produce small young plants. These small plants can be planted separately to form new plants. Such stems are called runners. Flowers propagated by runners include saxifrage, spider plant, etc.

The underground stems of some flowers are thickened, thick and long in appearance, and similar to roots. Such underground stems are called rhizomes. Rhizomes store abundant nutrients. They are similar to aboveground stems and have nodes, internodes, degenerated scale leaves, terminal buds, and axillary buds. Adventitious roots often arise at the nodes, and lateral buds develop there and can branch to form clumps. The clumps can be separated to form independent plants, such as canna, iris, aster, etc.

Bulbs refer to the underground stems of some flowers that are shortened and thickened, nearly spherical, with a flattened disc-like basal plate at the bottom, and scale leaves borne on the basal plate. Bulbs store abundant organic matter and water. Their terminal buds often produce true leaves and inflorescences. Axillary buds can develop between the scale leaves. Each year, one to several daughter bulbs can form from the axillary buds and separate from the side of the old bulb. Therefore, the multiplication rate can be increased by planting daughter bulbs separately. Examples include lily, tulip, hyacinth, narcissus, etc.

Corms: The underground stems of some flowers are shortened and thickened, nearly spherical, and store nutrients. Corms have nodes, degenerated leaves, and lateral buds. After the corm sprouts, a new corm forms at the base, and small cormels often grow beside the new corm. In production, new corms and cormels can be planted separately. Examples include gladiolus, freesia, arrowhead, etc.

Tubers: These are underground modified stems of perennial flowers. Their shapes vary, mostly nearly block-like, and they store certain nutrients. Roots arise from the base of the tuber. The top of the tuber usually has several sprouting points, and some bud eyes are also distributed on the surface of the tuber that can produce lateral buds. Tubers can be cut and divided for propagation. Such as grafting and layering methods

(1) Grafting method: Refers to the propagation method in which a part of one plant is grafted onto another plant, and after their tissues heal together, they are cultivated into an independent individual. This method is mostly used for flowering trees that are difficult to root by cuttings or difficult to obtain seeds from. Compared with seedlings, grafted flowering trees can bloom earlier and can maintain the excellent qualities of the scion. There are many flowering trees that can be propagated by grafting, such as osmanthus, chrysanthemum, cactus family plants, and so on.

(2) Layering method: That is, for branches close to the ground, soil is mounded at their base or their lower part is buried in soil; for higher branches, the cut part is wrapped with moist soil or moss, and after the branch roots, it is replanted to become an independent plant. This method is used for species that are difficult to propagate by other methods and can maintain the characteristics of the original cultivar. In greenhouse flowers, croton, white magnolia, camellia, bougainvillea, cordyline, etc. use this method.

IV. Tissue culture method

The process of inoculating a part of a plant, namely an explant, onto a culture medium through aseptic operation and culturing it under artificially controlled conditions to produce a complete new plant is called tissue culture. The theoretical basis of plant tissue culture is cell totipotency, that is, every cell of a plant has all the genetic information of that plant and the ability to develop into a complete plant.

Plant tissue culture is a new technology developed in the 20th century. In addition to its important value in basic theoretical research, it has increasingly demonstrated its enormous value in practical applications. According to the culture techniques already mastered, almost all isolated plant parts can be cultured and grown uniformly and consistently alone, and the culture conditions can be strictly controlled manually, which can exclude the influence of many unfavorable factors such as seasons. Therefore, in many countries and regions, it has become an important means of factory production and has been widely used in applied sciences such as agronomy, horticulture, medicine, and forestry, especially for some plants that do not set fruit or have low fruit set rates. The tissue culture method can greatly increase the propagation coefficient while saving labor.

Plant tissue culture can be divided into four stages:

The first stage is mainly to establish an aseptic system, that is, disinfection and inoculation of explants and culture media, to obtain callus or organs.

The second stage is mainly proliferation, continuously differentiating to produce new plants, or directly producing adventitious buds and embryoids. Subculture can also be repeated as needed to achieve the purpose of mass propagation.

The third stage is to transfer the plants for rooting culture. They can be transferred to rooting medium, or directly cut for cutting rooting.

The fourth stage is the acclimatization of test-tube plantlets, that is, the process of adapting test-tube plantlets to the external environment for a certain period after leaving the bottles.

Plant tissue culture requires certain sites, instruments, and equipment, roughly including a series of laboratories (aseptic room: an important place for inoculation; the area does not need to be large, but sealing and cleanliness must be ensured; chemistry laboratory: a place for preparing reagents and culture media; culture room: a place for culturing test-tube plantlets, equipped with culture racks, air conditioners, etc.; washing room: a place specifically for washing various glassware; sterilization room: mainly a place for disinfecting and sterilizing culture media, glassware, and tools; and cytology laboratory where a series of observations and analyses can be carried out, etc.), constant-temperature incubator, oven, refrigerator, balance, pH meter, autoclave, water distiller, microscope, etc., as well as some glassware (Erlenmeyer flasks, volumetric flasks, petri dishes, beakers, graduated cylinders, etc.).

Culture conditions vary depending on the plant material, but generally a temperature of 25+2°C, light intensity of 22000 lux, 8-10 hours of illumination per day, and a culture medium pH of 5.0---6.5 are used. Of course, adjustments can be made according to actual conditions.