Classification of ornamental crops by temperature requirements for cultivation

The thermal regime directly determines the intensity of photosynthesis, transpiration, respiration, and morphogenesis of ornamental crops. To obtain high-quality cut flowers and viable nursery plants, an agronomist must adapt the technology to the biological requirements of the plants. Prolonged lack or excess of heat inhibits physiological processes and can lead to the death of the plantings.

Temperature groups of floral crops

All floral and ornamental crops are divided into open-field and protected-ground plants. Based on their heat requirements, open-field species are divided into three main categories: heat-loving, cold-hardy, and winter-hardy. It is important for an agronomist to clearly distinguish these groups when planning crop rotation and planting dates.

Heat-loving crops (marigolds, begonia, zinnia) during the growing season cannot tolerate even short-term temperature drops to 0...1 °С. This group also includes greenhouse plants that are planted in flower beds in summer: agave, Sprenger asparagus, chlorophytum. Cold-hardy species (verbena, snapdragon, ornamental kale) are able to tolerate low positive temperatures of 1...3 °С and short frosts down to –1...–3 °С without losing their marketability. Winter-hardy perennials (iris, peony) successfully overwinter in the soil, withstanding frost, rotting under snow at 0 °С, root ruptures, and soil heaving.

Protected-ground crops require different temperature regimes depending on the season and the developmental phase. During the spring-summer period, they are divided into heat-loving (requiring 18...20 °С and above) and moderate (requiring no more than 16...18 °С, for example, calla). In winter, most plants enter a dormant state, so their need for heat decreases.

Certain greenhouse plants exhibit high temperature plasticity in winter. Aloe, dracaena, ficus, and epiphyllum can be grown in both warm and moderately warm sections during this period. This allows for the optimization of the farm's greenhouse heating costs.

Managing the thermal regime in the field and greenhouses

Each species of ornamental plants has its own temperature thresholds for the start, peak, and cessation of growth. Violating these limits stops the growing season and reduces the productivity of the nursery. The main temperature indicators of crops are as follows:

• Minimum temperature for growth of heat-loving plants — more than 10 °С
• Minimum temperature for growth of cold-hardy plants — 5 °С
• Optimum temperature for growth of heat-loving plants — 30–35 °С
• Optimum temperature for growth of cold-hardy plants — 31 °С
• Maximum temperature for most plants — 37–44 °С
• Maximum temperature for southern plants — 50 °С

In the open field, an agronomist can artificially adjust the microclimate of the site. For heat-loving crops, light soils on southern slopes are selected, protected from winds by buildings or hedges. In case of a threat of late frosts, plastic covers are used, and irrigation and smudging are performed.

It is possible to increase plant resistance to temperature stress by treatment with physiologically active substances. Another effective technique is the pre-hardening of greenhouse seedlings at moderate temperatures and humidity before planting. Under protected-ground conditions, continuous monitoring and regulation of the air, soil, or substrate temperature are necessary.

Temperature regime: regulating growth and preventing stress

Air temperature is a key tool for managing the growing season of ornamental crops. Sharp fluctuations and overcooling disrupt metabolism and delay biosynthesis in plant tissues. To optimize the development of crops in protected ground, follow the basic rules of the temperature regime:

• Maintain the night temperature 2–3 °C lower than the day temperature.
• Lower the temperature in greenhouses in summer and on sunny days.
• During the dormant period, keep the temperature lower than during active vegetative and reproductive growth.
• In winter, under a natural daylight duration, maintain a lower temperature than in similar phases of development in summer.
• In the reproductive growth phase (winter and spring), keep the temperature at the level of the vegetative phase or higher.
• With artificial lighting, keep the temperature at the level of the base rate or higher.

• Day/night temperature difference — 2–3 °C
• Bud forcing temperature — 20 °C
• Flowering inhibition temperature — 10–15 °C
• Optimal CO2 concentration for photosynthesis — 0.1–0.3%
• Oxygen content in the air — 21%

To manage flowering times and cut flower quality, temperature regime is adjusted in conjunction with humidity. In protected soil, parameters are corrected by changing heating intensity, ventilation, and air conditioning. Spraying, irrigation, and shading are used additionally.

1. To stimulate tulip and narcissus bud opening during forcing, raise the greenhouse temperature to 20 °C.
2. After the buds show color, lower the temperature to 10–15 °C to prolong flowering and extend the product marketing period.

When planning the microclimate, rely on technical standards for specific crops. The table below lists optimal temperature parameters for carnations, roses, and chrysanthemums depending on lighting and development stage.

Optimal air temperature in greenhouses for growing certain flower crops, °С

Gas composition of air: how to avoid carbon dioxide deficiency

Besides heat, the gas regime is critically important for plant development. Oxygen, which accounts for about 21% of the air, ensures oxidation processes and energy release in cells. Carbon dioxide is essential for photosynthesis. Its concentration in the air above crops fluctuates from 0.28 to 0.61 mg/l, rising in areas with high organic content.

Plants reach maximum photosynthesis intensity at a carbon dioxide concentration between 0.1–0.3% (at a natural level of about 0.03%). In greenhouses, active CO2 consumption begins in February–March and continues until winter, and under supplemental lighting, it occurs round-the-clock. Gas released by the substrate is quickly exhausted by the plantings during daylight hours, creating an acute nutrient deficiency.

To replenish carbon dioxide in open and protected soil, the following agricultural techniques are used:

• Application of organic fertilizers (manure).
• Application of mineral calcium, sodium, magnesium, and potassium carbonates.
• Use of ammonium, potassium, and calcium bicarbonates.
• Application of carbon-ammoniacates (solutions of ammonium carbonate or a mixture of ammonium carbonate with urea in ammonia water).

Carbon dioxide top dressing technology and air exchange regulation

Additional saturation of the air environment with carbon dioxide directly affects the productivity of flower crops. In greenhouse farming, this technique allows for increasing flower yield by 20–35% and significantly improving their quality. Furthermore, carbon dioxide top dressing accelerates the start of flowering and increases the yield of cuttings from carnation and chrysanthemum stock plants.

• Flower yield increase — 20–35%
• Coefficient for calculating CO₂ application rate — 0.014
• Air exchange rate per hour — from 5 to 20 times
• Air movement speed in the plant zone — up to 3 m/s

The most effective method of supplying carbon dioxide is burning propane or a propane-butane mixture in gas generators. The resulting warm CO₂ is fed into the greenhouse through a gas distribution system along with water vapor, which also participates in the photosynthesis process. The daily carbon dioxide requirement P (in kilograms) is calculated by multiplying the greenhouse area F (in square meters) by the coefficient 0.014. Liquid carbon dioxide or dry ice are used as alternative sources of carbon dioxide.

Regular air exchange in greenhouses is necessary for normal plant respiration and photosynthesis processes. Oxygen enters the protected soil through ventilation and forced air circulation. Along with maintaining optimal humidity and carbon dioxide concentration, constant air exchange is created in greenhouses. This exchange rate should be between 5 and 20 times per hour.