Classification and morphological features of ornamental flowering crops for nurseries

To plan work in the nursery and flower gardens ornamental crops are classified according to their biological characteristics, agricultural techniques, and landscaping use. The main guide for an agronomist is the plants' heat requirement. Based on this, all crops are divided into two groups: open-ground and protected-ground.

Open-ground crops are most decorative in areas unprotected from the external environment. These include plants from temperate latitudes, subtropical species from the Mediterranean and East Asia, as well as certain tropical plants from India and South America. Calendula, cosmos, poppy, and California poppy can be sown directly into the soil — they bloom profusely and produce mature seed.

All ornamental plants of the open ground are subdivided into four main groups:

• annuals;
• biennials;
• perennial herbaceous;
• flowering shrubs.

Protected-ground crops require a regulated microclimate and high temperature. These are mainly heat-loving herbaceous and woody species from the tropics, subtropics, deserts, and semi-deserts, such as the coffee tree and palms. In temperate latitudes, they are grown in greenhouses and rooms, but in summer, some of them can be kept outdoors.

The following categories of plants are distinguished in this group:

• forcing plants;
• seasonally flowering;
• evergreen.

According to decorative characteristics, crops are divided into flowering, ornamental-foliage, and species with unique fruits. In practice, the purpose of the nursery plant material determines its cultivation scheme. Plants are produced for landscaping objects, obtaining cut flower or pot products, as well as for use as stock plants for harvesting seed and vegetative material.

The role of vegetative organs in plant life

The development of ornamental crops depends directly on the state of their vegetative organs — the root, stem, and leaf, which are already formed in the seed embryo. They perform life-support functions: mineral nutrition, photosynthesis, respiration, and vegetative propagation. This group also includes organ modifications (metamorphoses), such as bulbs, tubers, and rhizomes.

The root is an axial organ of unlimited growth with positive geotropism. Its main tasks are the absorption of water with nutrients and anchoring the plant in the substrate. At the same time, some ornamental crops, along with ordinary roots, can develop modified forms.

Specifics of the root system: from storage to fixation

The type of root system determines the planting depth, irrigation regime, and propagation methods of ornamental crops. Adventitious and lateral roots of plants often modify, taking on special functions. For example, dahlia and dame's rocket form fleshy root tubers (cones), which accumulate nutrients and require careful handling during digging. In climbers, such as common ivy, attachment roots form, which securely hold shoots on vertical surfaces of walls or tree trunks.

Epiphytic plants (Monstera, orchids, Tetrastigma) form hanging aerial roots of brown or yellow color. A specialized spongy tissue — velamen — develops on their surface, capable of absorbing and retaining atmospheric humidity. In some tropical orchids, such roots are flattened, contain chloroplasts, and fully participate in photosynthesis. The ability to form aerial roots is retained in epiphytes even when grown in greenhouse conditions or protected ground.

For crops growing in waterlogged coastal zones and swamps, stilt roots are characteristic. In some species of palms and the banyan fig, they develop in the lower part of the trunk and take on an arched shape. Such structures lift the above-ground part of the plant above water, protecting the trunk from flooding during high tides.

Morphological features of stems and leaves

The stem serves as an axial vegetative organ with negative geotropism and unlimited growth. It bears buds, leaves, flowers, and fruits, performing supporting, conducting, storage, and protective functions. In seed reproduction, the stem develops from the embryonic plumule, and in vegetative reproduction, from buds on a cutting, rhizome, or corm. Stem growth in height usually occurs due to cell division of the apical bud. However, in crops with a basal leaf rosette (aquilegia, astilbe, gerbera, calceolaria, primula), as well as in ornamental grasses (blue fescue, foxtail barley), intercalary growth is observed, during which a leafless scape-shoot elongates due to cell division at its base.

Stem branching determines the habit of the bush and the density of the canopy. Shoots of the first order develop from the buds of the main stem, on which shoots of the second and subsequent orders then form. In woody plants (hydrangea, palm, rose, lilac), the stems are perennial, lignifying, with active meristematic tissue — the cambium. In herbaceous species, the cambium exists in a rudimentary form or is absent, and their shoots usually die off at the end of the season.

Leaf color is a key ornamental trait when selecting an assortment. The sizes of above-ground organs and their lifespans vary greatly depending on the biological group of the plant, which must be taken into account when planning plantings and assessing the marketability of crops. The main morphometric parameters of stems and leaves of popular ornamental crops are shown in the table below.

Leaf apparatus: classification by ornamental traits and metamorphoses

The leaf apparatus of ornamental crops determines their marketability and care requirements. In practical floriculture, the coloration of the leaf blade is divided into six main types:

• solid-colored — green on both sides;
• colored — any tone except green;
• variegated — with different colors on the upper and lower sides;
• spotted — with contrasting spots on a background;
• multicolored — with multi-colored sectors on a single blade;
• bordered — with a distinct border of a different color along the edge of the leaf.

The consistency and surface character of a leaf directly affect the plants' resistance to environmental conditions. Herbaceous (thin and soft) leaves require strict humidity control, whereas leathery and fleshy (succulent) ones tolerate short-term moisture deficits. A waxy bloom and pubescence (hairiness) provide additional protection against evaporation and pathogens, while bare, glossy, or matte leaves are more sensitive to external influences.

Depending on growing conditions, leaves can undergo metamorphosis to perform special functions. In cacti, they have turned into protective spines, and in sweet pea, into support tendrils for anchoring to a trellis. Such parts of a flower as sepals, petals, stamens, and pistils also have a leaf origin and are responsible for protection, attracting pollinators, and reproduction. The dense scales of hyacinth, narcissus, and tulip protect the renewal buds and store nutrients.

Underground shoots: specifics of structure and storage

Underground modified shoots serve plants for overwintering, storing nutrients, and vegetative reproduction. A rhizome is a long-lived shoot with a lifespan of 2 to 25 years or more. It grows horizontally in aspidistra, hybrid iris, and chrysanthemum, or vertically in zantedeschia, Siberian iris, and primula. Unlike roots, the rhizome features nodes with buds and underdeveloped leaves, and alstroemeria additionally uses it to store starch and sugars.

For accelerated propagation, stolons and caudexes are used. Fast-growing underground stolons of zantedeschia, crocus, lilac, tulip, freesia, and chrysanthemum are easily separated from the mother plant and bear a bud at the tip for a new plant. The caudex (stem-root) of heuchera, delphinium, peony, and phlox paniculata does not separate, but forms many renewal buds and accumulates nutrients together with the root.

Stem tubers and bulbs are the main propagation organs of many valuable crops. Tubers of tuberous begonia, gloxinia, and cyclamen are formed from the hypocotyl, and their buds are concentrated on the apical (top) part. In bulbs, the stem is shortened to a basal plate, to which succulent scales storing carbohydrates are attached. Lateral buds develop in the axils of these scales, producing bulbils.

Corms of gladiolus, crocus, and freesia combine the properties of a tuber and a bulb. Nutrients in them are stored in the thickened base of the stem, which is protected on the outside by dry scales of dead leaves. Renewal buds are formed in the axils of these scales, and the main flowering shoot develops from the largest bud at the top of the corm.

Above-ground shoot modifications and their practical significance

Above-ground shoots of ornamental crops are often modified to adapt to the external environment or for propagation. In arid conditions, the main task becomes the accumulation of moisture. Plants with succulent water-storing organs are called succulents. Depending on the localization of water reservoirs, they are divided into stem and leaf varieties.

• Stem succulents (cacti, euphorbia) — their stems consist almost entirely of water-storing tissue, which also participates in photosynthesis.
• Leaf succulents (aloe, houseleek, stonecrop, jade plant) — retain moisture reserves in fleshy leaves.

Besides water storage, above-ground modifications perform functions of protection, climbing support, and vegetative propagation. Knowledge of these features helps the agronomist correctly select methods of crop propagation and design support structures in greenhouses. Thus, runners and stolons require control of the spread area, and bulbils serve as ready-made planting material. The main types of such modifications are given below:

• Phylloclades — stems or entire shoots that perform the function of leaves and have their shape (ferns, butcher's broom).
• Cladodes — modified stems without a characteristic leaf shape (asparagus).
• Thorns — protective organs of stem (rose) or leaf (cacti) origin, developing during moisture deficit.
• Tendrils — modified lateral shoots for fixation on supports (tetrastigma, cissus).
• Runners — stems creeping horizontally, rooting at the nodes (periwinkle).
• Stolons — shoots with long internodes, rooting via terminal buds to form rosettes (ajuga reptans).
• Bulbils — above-ground shoots in the axils of leaves (tiger, white, bulbiferous lilies) or inflorescences, serving for vegetative propagation.

• The share of water-storing cells in cactus stems — 90%
• Yield reduction of seeds without pollinators — by 2–3 times
• Timing for pollen collection for artificial pollination — the day before or well in advance

Flower morphology and pollination process management

The flower is the main organ determining the ornamental value of plants, with double forms being especially prized in gardening. Most often, doubleness arises from the transformation of stamens into petals (poppy, rose), less often — from an increase in perianth segments (lily, tulip) or splitting of petals (fuchsia). Since the petals of most plants originated from stamens, only individual stamens remain in the double flowers of azalea, carnation, and peony. Most crops form bisexual flowers, however, unisexual species are also encountered (castor bean, wax and tuberous begonia). Flowers can be located singly at the tips of shoots (globeflower) or in leaf axils (clarkia, lobelia), or be gathered into inflorescences.

Inflorescences increase the total duration of flowering and improve the chances for successful pollination. By the nature of axis growth, they are divided into determinate (sympodial), indeterminate (monopodial), and mixed. In mixed types, simple inflorescences are combined into complex structures. For example, in ageratum, capitula are gathered into umbels, in goldenrod — into a panicle, and in yarrow — into complex corymbs.

Most ornamental crops require cross-pollination. To avoid self-pollination, plants use mechanisms of asynchronous maturation of organs or self-sterility, where their own pollen germinates poorly on the stigma (lobelia, petunia, mignonette, salvia). Pollen of entomophilous species is carried by insects. In this case, diurnal flowers attract them with bright coloring, while nocturnal ones (sweet alyssum, stock, flowering tobacco) intensify their fragrance after dark and have light-colored petals. In nature, some species, such as alstroemeria, can be pollinated by birds (hummingbirds, sunbirds).

The success of artificial pollination and seed production of ornamental crops depends on pollen viability. Its storage period can be extended by maintaining a cool temperature regime. Under optimal conditions, pollen retains fertility from several days to a year.

• Pollen storage temperature — 5–10 °C
• Rose pollen viability — 22 days
• Gloxinia and stock pollen viability — up to 40 days
• Clivia pollen viability — up to 1 year

For guaranteed fruit set during artificial hybrid pollination, it is important to accurately determine the readiness of the maternal plant and follow the pollen transfer technique.

1. Wait for the pistil stigma on the maternal plant to mature — the appearance of moisture on its surface serves as a signal of readiness.
2. Apply the prepared pollen to the stigma using a soft brush or a small piece of cotton wool.
3. When working with large flowers, carefully place mature anthers directly into the female flower.

When organizing seed production plots, the pollination method of the crop is taken into account. Wind-pollinated (anemophilous) plants, such as amaranth, castor bean, wormwood, ornamental grasses, and sedges, form inconspicuous flowers with light, volatile pollen and feathery stigmas. They require strict spatial isolation. Self-pollinating crops (annual aster, stock, sweet pea) are less demanding regarding the distance between plantings. In some of them, for example in wood sorrel, the flowers do not open at all before self-pollination (cleistogamy), which prevents contamination of the cultivar.

Types of fruits and calibration of seeds of flowering ornamental crops

Most ornamental plants form dry fruits: follicles, multi-follicles, legumes, siliques or silicles, capsules, nuts, nutlets, achenes, and caryopses. A succulent pericarp is formed only in a few crops, such as asparagus, lily of the valley, physalis, and fuchsia. In pineapple and fig, multiple fruits are formed — individual fruits fused together or the entire inflorescence as a whole.

Seeds of floral crops vary greatly in shape, color, and surface texture — it can be smooth, grooved, ribbed, tuberculate, or spiny. The size and mass of the seeds determine the sowing rate and the choice of sowing discs. Even within a single inflorescence, seeds develop unevenly, which is why they are calibrated before sowing.