Plant Environmental Factors

Derived from the Idaho Master Gardener Program Handbook: Chapter 2 Intro to Horticulture & Plant Physiology

  1. Horticulture defined: "the science and art of growing fruits, vegetables, and flowers."
  2. Climate in Horticulture
    1. Macroclimate: "the long-term weather patterns of a large geographical area"
    2. Microclimate: your yard may have several microclimates!
      1. Low areas can collect denser cold air
      2. Slopes can be warmer, esp south facing
      3. North slopes are wetter but cooler
      4. Leeward sides of hills warmer
      5. Houses, fences, sidewalks absorb heat
      6. Bodies of water moderate temperatures
      7. Elevation results in lower temperatures. 300ft= 1°F cooler
      8. Raised beds heat and cool quicker.
      9. Large plants create windscreens & shade.
      10. Sandy soils warm quicker than clay.
  3. Role of Temperature in Horticulture
    1. "Temperature is the climatic factor that, more than any other, determines the kinds of plants that will grow in an area."
    2. "Photosynthesis, transpiration, and respiration increase with rising temperature."
    3. "Each plant type has an optimal temperature needed for growth."
    4. "Some plants prefer cooler nights or days, whereas others prefer warmer nights or days."
    5. Cool-season crops examples: sweet peas, pansies, garden peas, onions, carrots, potatoes, lettuce, cabbage, and broccoli.
      • Seeds of cool-season crops will germinate at a soil temperature of just 40°F.
    6. Warm-season crops examples: sweet corn, tomatoes, peppers, melons, zinnias, and marigolds.
      • Seeds of warm-season crops require a soil temperature of 60°F or higher to germinate
    7. High Temperatures
      1. Plants tend to prefer warmer temperature, but note the actual temperature depends on the nature of the plant.
      2. "Most temperate zone perennials need a cold period to overcome their physiological dormancy, or rest period..."
      3. Plant pests and diseases tend to prefer warmer temperatures.
    8. Low Temperatures
      1. Bolting can occur from extended cool temperatures or early summer heat. Food crops are often rendered inedible.
      2. Dormancy of perennials (hardening off) is assisted by developing cold, short days and freezing.
      3. Winter damage to plants is increased by lack of water in their tissues or the soil. Frozen soil seems as a desert.
      4. Frost heave can damage young plant roots.
      5. Spring frosts damage transplants and opened buds.
    9. Temperature Modification
      1. Modifying high temps can be done with shading to limited effect.
      2. Modifying low temps
        1. Plastic or fabric canopies will trap the ground's heat giving you a 2° to 6°F gain. Cold frames are more permanent.
        2. Mulch is a protective cover placed on the ground around plants. Organic (straw) or inorganic materials can be used. Mulching will protect the plant crowns.
        3. Heaters and sprinklers are used in commercial operations.
  4. Role of Light in Horticulture
    1. Light Quality
      1. Various parts of the spectrum affect plants.
      2. Incandescent lights emit too much infrared.
      3. Blue encourages vegetative growth. Fluorescent works for seedlings.
      4. Flowering plants like more red wavelengths.
    2. Light Intensity:
      1. Full sun at noon in summer = 12,000 foot-candles
      2. Overcast day in winter here = 1000 - 5000 foot-candles
      3. Well-lit house = 50 - 300 foot-candles.
      4. Insufficient light results in elongation.
    3. Light Duration
      1. Flowering response controlled by length of uninterrupted darkness.
      2. Long-day plants: short nights. Examples: Onions, spinach
      3. Short-day plants: long nights. Examples: Christmas cactus, poinsettia, chrysanthemums
      4. Day-neutral plants: