Ornamentals
Senior 3 · Agriculture
Ornamentals
Lesson 2 of 3
25 min
Let's begin
A nursery owner in Musanze has one beautiful red hibiscus bush, and customers want fifty plants exactly like it. If she sows its seed, the seedlings may flower pink, white or a paler red, because seed mixes the features of two parents. If she cuts fifty short pieces of stem and roots them, every new plant will be a copy of the bush she admires. Choosing the right way to multiply a plant is the first skill of anyone who grows ornamentals for sale.
Definition
Propagation: producing new plants from a parent plant.
Sexual propagation: raising plants from seed, which is formed when pollen fertilises an ovule; the offspring vary.
Vegetative (asexual) propagation: raising new plants from a part of the parent (stem, leaf, root or storage organ). The new plants are a clone: genetically identical to the parent, or "true to type".
Cutting: a piece of stem, leaf or root that is made to grow roots and become a new plant.
Layering: making a stem form roots while it is still attached to the parent; in air layering (marcotting) the roots form in a ball of moist moss on the branch.
Division: splitting a clump into pieces, each with roots and shoots.
Rootstock and scion: in budding and grafting, the rooted plant that provides the roots (rootstock) and the bud or shoot of the wanted variety that is joined to it (scion).
Seed is used for annual flowers such as marigold, zinnia and cosmos, and for many ornamental trees, such as jacaranda, African tulip tree and cypress.
Advantages: cheap; many plants at once; seed is easy to store and carry; plants grown from seed have a strong tap root; breeders get new colours from seed. Disadvantages: seedlings of hybrids are not true to type; trees from seed take years to flower; some seed is dormant and hard to germinate; double-flowered varieties often set no seed at all.
| Method | How it is done | Ornamentals propagated this way |
|---|---|---|
| Stem cuttings | pieces of stem 10 to 30 cm long, rooted in moist medium (Figure 5a); hardwood (mature stems), semi-hardwood or softwood (young tips) | bougainvillea, hibiscus, duranta, acalypha, roses, croton |
| Leaf cuttings | a leaf or a piece of leaf set upright in moist sand | snake plant (Sansevieria), begonia, kalanchoe |
| Layering | a low branch pegged into the soil (simple layering), or a ring of bark removed and wrapped in moss (air layering, Figure 5b) | bougainvillea, jasmine (simple); ficus, croton (air) |
| Division | a clump dug up and split, each piece with roots and shoots | agapanthus, canna, bird of paradise, ferns; lawn grass as sprigs |
| Storage organs | planting bulbs, corms, rhizomes or tuberous roots, and the small ones that form beside them | lilies (bulbs), gladiolus (corms), canna (rhizomes), dahlia (tuberous roots) |
| Suckers and offsets | small plants that grow from the base are cut off with some roots | aloe, agave, bromeliads |
| Budding and grafting | a bud (budding) or shoot (grafting) of the wanted variety is joined onto a hardy rootstock | roses, especially for cut flowers |
| Tissue culture | tiny pieces of plant grown on a sterile nutrient jelly in a laboratory | large numbers of identical plantlets for flower farms |
Figure 5. (a) Preparing and setting a stem cutting. (b) Air layering a branch.
Choose a branch about as thick as a pencil or a finger. Remove a ring of bark 2 to 3 cm wide, scraping off the soft green layer beneath. Wrap the wound in a fist-sized ball of moist moss or soil, cover it with clear polythene, and tie both ends. Removing the bark cuts the phloem, so sugars and hormones moving down from the leaves collect above the ring, and roots grow there. The wood (xylem) is not cut, so water still reaches the leaves and the branch stays alive. When roots fill the ball (often after six to ten weeks), cut the branch off below it and pot it.
A hardy rootstock is grown first. A T-shaped cut is made in its bark near the ground, a shield-shaped bud of the wanted variety is slipped under the bark, and the joint is tied firmly with tape. When the bud has joined and starts to grow, the rootstock's own shoot above it is cut away, so all the plant's energy goes into the new variety.
Why propagate vegetatively? The new plants are true to type, flower sooner than seedlings, and it works for plants that set little or no seed. Drawbacks: diseases, especially viruses, pass from parent to offspring; each parent gives fewer new plants than seed would; and budding, grafting and tissue culture need skill and equipment.
Worked example 1: how many cuttings to set?
Problem. Last season a nursery set 120 bougainvillea cuttings and 96 rooted. (a) Find the rooting percentage. (b) A customer orders 300 rooted plants. How many cuttings should be set?
Solution.
Answer: 80%; set 375 cuttings (a few more, as a margin).
Worked example 2: how fast does division multiply a plant?
Problem. A canna clump can be split into 5 good pieces each year, and each piece grows into a clump that can be split into 5 again the next year. Starting from one clump, how many plants are there after 3 years?
Solution. Each year multiplies the number by 5: After 3 years: .
Answer: 125 plants, all identical to the first clump.
Warning
Common mistakes. (1) Planting a cutting upside down: mark the base end as you cut. (2) Leaving many leaves on a cutting: it wilts and dies before it can root. (3) Keeping the medium waterlogged: the base rots. (4) Taking cuttings from a sick plant: the disease goes into every new plant. Use a clean, sharp blade, and take care with knives.
Tip
Memory hook: "Seed for many, cuttings for copies." Seed gives many varied plants cheaply; every vegetative method (cuttings, layers, divisions, corms, buds) gives copies of the parent.
Gardening Australia: "Softwood, hardwood and semi-hardwood cuttings". Watch where each cut is made, how the leaves are trimmed, and how the cuttings are set in the mix.
Practice
Key points
Sources and credits
Written by Ganzaa: the text, the table, Figure 5 and the worked examples.
This lesson adapts CC BY-SA material and is shared under CC BY-SA 4.0.
