Growing and Caring for Ornamentals – Ornamentals (Senior 3 Agriculture) | Ganzaa

Ornamentals

Lessons

Senior 3 · Agriculture

Ornamentals

3 lessons

Lesson 3 of 3

Growing and Caring for Ornamentals

25 min

Let's begin

A flower bed planted in September can look wonderful in November and be a tangle of dead stems, weeds and leggy shoots by March. The roundabouts and road medians of Kigali stay neat all year only because gardeners water, weed, feed, trim and mow them on a schedule. Planting an ornamental is quick; caring for it is the real work. This lesson follows the practices in order: raising plants in a nursery, planting them out, routine care, pruning, lawns, pests and diseases, and cut flowers.

Definition

Potting mix: the soil mixture used in pots and nursery sleeves, commonly 2 parts topsoil, 1 part well-rotted compost or manure and 1 part sand by volume.

Mulch: a layer of dry grass, leaves, wood chips or similar material spread on the soil to keep in moisture, smother weeds and keep the soil cool.

Pruning: cutting off parts of a plant to keep it healthy, shape it or make it flower better. Deadheading removes faded flowers; pinching removes the growing tip so that side shoots grow and the plant becomes bushy.

Topiary: clipping shrubs into neat geometric or animal shapes.

Sprigging: establishing a lawn by planting small rooted pieces (sprigs or splits) of a creeping grass.

1. In the nursery

Fill polythene sleeves or pots with potting mix; make sure they have drainage holes. Keep young plants under light shade, water them daily in dry weather, and weed the sleeves. Space the sleeves out as the plants grow, so that they do not grow tall and thin. Harden off plants for one to two weeks before planting out, by giving more sun and less water.

2. Planting out

Choose the site for the plant: sun or shade, good drainage, and room for its full adult size. Keep large trees well away from buildings, walls, water pipes and power lines.
Dig a hole wider than the root ball; for trees and large shrubs about 60 cm wide and deep. Mix the topsoil with well-rotted manure or compost.
Plant at the same depth as the plant grew in its sleeve. Cut and remove the polythene, firm the soil, water well, and mulch. Stake young trees and protect them from goats.
Hedges are planted in a trench, one row or two staggered rows, with plants 30 to 45 cm apart.
Flower beds: dig deeply, add compost, and plant tall flowers at the back of a border (or in the middle of a round bed) and short ones at the front.

3. Routine care

Watering: water deeply and less often, in the early morning or the evening, at the base of the plant. Roots then grow deep. Leaves left wet overnight encourage fungal diseases.
Mulching and weeding keep moisture in and competition out.
Feeding: compost or manure once or twice a year; small amounts of a balanced fertiliser such as NPK 17-17-17, scattered round the plant (not against the stem) and watered in; a weak liquid feed for pots.
Staking and tying tall flowers such as gladiolus and climbers on their supports.

4. Pruning and training

First remove the "three Ds": dead, diseased and damaged shoots.
Cut with sharp, clean secateurs, just above an outward-facing bud, at a slight slant away from the bud so rain runs off.
Deadhead annuals and roses so they keep flowering instead of setting seed; pinch young marigolds and chrysanthemums to make them bushy.
Hedges are trimmed often, a little at a time, and always kept wider at the base than at the top (Figure 6).

Figure 6. Cross-sections of a well-trimmed and a badly trimmed hedge.

5. Lawns

Dig, remove stones and weed roots, level and firm the ground; add compost.
Plant sprigs of a creeping grass such as Kikuyu grass or paspalum 10 to 15 cm apart (or lay turf, or sow Bermuda grass seed), at the start of the rains.
Water lightly and often until the grass has spread; pull out weeds by hand.
Mow for the first time when the grass is about 8 cm tall. Follow the one-third rule: never cut off more than one third of the leaf height at one mowing. Keep the lawn at about 3 to 5 cm.
Trim the edges, spread a little compost (top dressing) in the rains, and repair bare patches with new sprigs.

6. Pests and diseases

Problem	Signs	Control
Aphids	clusters of small soft insects on young shoots; curled leaves; sticky honeydew	wash off with a strong jet of water or soapy water; encourage ladybirds
Mealybugs and scale insects	white, cottony or waxy masses on stems and under leaves; plant weakens	cut off badly infested shoots; wipe or wash off; approved insecticide only if severe
Red spider mites	fine webs, pale speckled leaves, worst in dry weather	spray leaves with water; remove badly affected leaves
Powdery mildew	white powder on leaves	space plants for air flow; remove affected leaves; resistant varieties
Black spot of roses	black spots with yellowing; leaves drop	collect and destroy fallen leaves; water the soil, not the leaves
Root rot	wilting even though the soil is wet; brown, soft roots	good drainage; water less; pots with drainage holes

Inspect plants every week: a problem caught on one plant is easily stopped. Pesticides are used only when needed, by trained adults, following the safety rules of Unit 2.

Pink hibiscus mealybugs crowded on a shoot. The insects are pink under a coat of white wax, and the white cottony masses hold their eggs. They suck sap and make shoots twisted and stunted. Photo: CSIRO, public domain, via Wikimedia Commons

7. Cut flowers and pot plants

Cut flowers are harvested in the cool of the early morning, at the right bud stage (roses when the bud is just starting to open). Put the stems into clean water at once, remove leaves that would sit below the water line, recut the stem ends at a slant, and keep the flowers cool. Export farms keep roses cold from the greenhouse to the aircraft: Rwanda's flowers reach buyers in the Netherlands and the United Kingdom (MINAGRI, 2025).

Pot plants need pots with drainage holes, water only when the top of the mix is dry, turning so that all sides get light, and repotting into a larger pot when roots circle the inside of the pot.

Worked example 1: mixing potting mix

Problem. A school nursery will fill 60 sleeves, each holding 1.5 L, with a mix of topsoil, compost and sand in the ratio 2 : 1 : 1. How much of each is needed?

Solution.

Total volume $= 60 \times 1.5 = 90\,\mathrm{L}$ .
The ratio has $2 + 1 + 1 = 4$ parts, so one part $= \frac{90}{4} = 22.5\,\mathrm{L}$ .
Topsoil $= 2 \times 22.5 = 45\,\mathrm{L}$ ; compost $= 22.5\,\mathrm{L}$ ; sand $= 22.5\,\mathrm{L}$ .

Answer: 45 L of topsoil, 22.5 L of compost and 22.5 L of sand.

Worked example 2: the one-third rule

Problem. A lawn has grown to 9 cm during the school holidays. The groundsman wants it at 4 cm. How should he mow it?

Solution. At each mowing he may remove at most one third, leaving at least two thirds.

First mowing: $9 \times \frac{2}{3} = 6\,\mathrm{cm}$ .
A few days later: $6 \times \frac{2}{3} = 4\,\mathrm{cm}$ .

Answer: two mowings a few days apart, first to 6 cm and then to 4 cm. Cutting straight to 4 cm would remove more than half the leaf, and the lawn would turn brown and let weeds in.

Warning

Common mistakes and safety. (1) Planting too deep or leaving the polythene sleeve on the roots. (2) Watering a little every day: roots stay shallow. (3) Trimming a hedge flat-topped and wider at the top. (4) Mowing too short. Keep secateurs and shears closed when not in use, carry them pointing down, and keep hands and feet clear of mower blades; switch off a motor mower before clearing grass from it.

Tip

Exam technique: when asked for the "cultural practices" of an ornamental, list them in order (raise, harden, plant, water, mulch, weed, feed, prune, protect) and give the reason for each.

✦

Practice

What is hardening off, and why is it done?
Give four points to consider when choosing where to plant an ornamental tree.
Explain why a hedge should be trimmed wider at the base than at the top.
Distinguish between deadheading and pinching.
State the one-third rule for mowing. A lawn is 7.5 cm tall: what is the lowest height to which it may be cut in one mowing?
A lawn of 6 m by 5 m is to be sprigged with Kikuyu grass at 15 cm by 15 cm. About how many sprigs are needed?
For each sign, name the likely problem: (a) white cottony masses on hibiscus stems; (b) fine webs and speckled leaves in dry weather; (c) black spots on rose leaves.
A gardener waters his roses every evening by spraying the leaves, and they are covered in black spots. Evaluate his practice and advise him.

Show answersHide answers

Gradually giving nursery plants more sun and less water for one to two weeks before planting out, so that they toughen and survive the harsher conditions of the open ground.
Any four: sun or shade needed; drainage; room for the full adult size; distance from buildings, walls, pipes and power lines; purpose (shade, screen, avenue); soil.
Light then reaches every part of the sides, so the lower branches keep their leaves and the hedge stays dense to the ground; a top-heavy hedge shades its own base, which goes bare and gappy.
Deadheading removes faded flowers, so the plant keeps flowering instead of setting seed; pinching removes the growing tip, so side shoots grow and the plant becomes bushy.
Never remove more than one third of the leaf height at one mowing. $7.5 \times \frac{2}{3} = 5\,\mathrm{cm}$ .
Area $= 6 \times 5 = 30\,\mathrm{m^2}$ ; sprigs per square metre $= \frac{1}{0.15 \times 0.15} \approx 44.4$ ; total $\approx 30 \times 44.4 \approx 1\,333$ sprigs.
(a) mealybugs; (b) red spider mites; (c) black spot of roses.
Poor practice: leaves left wet overnight favour fungal diseases such as black spot. He should water at the base of the plants in the early morning, water deeply and less often, collect and destroy fallen spotted leaves, and space or prune the bushes so air moves through them.

Key points

Raise plants in potting mix, harden them off, and plant them at the same depth in a well-prepared hole, then water, mulch and stake.
Water deeply at the base, mulch, weed and feed; prune the three Ds first, deadhead and pinch; keep hedges wider at the base.
Lawns: sprig a creeping grass, mow by the one-third rule to 3 to 5 cm; inspect plants weekly for pests and diseases.

11. In the nursery

22. Planting out

33. Routine care

44. Pruning and training

55. Lawns

66. Pests and diseases

77. Cut flowers and pot plants