Greenhouse
Environments and controls
Plants respond to environmental conditions. Frequently outside there is not the right environment for the kind of plants you want to grow. A greenhouse is to provide the optimum environment for plant growth.
Basic plant needs
· Water
· The correct temperature range
· Light
· Air exchange
· Adequate medium and nutrition
Temperature optimum
· Best rate of growth
· Best quality product
· Highest economic return
· This is a real trick when you have a variet of plants!
Considerations for temperature control
· Different growth stages = different temperatures (see handout #1)
· Some plants have a very narrow tolerance range
· Radiant energy from sun is extremely variable day to day, season to season and even hour to hour.
o Thus daytime temperatures are hard to control
o But night time temperatures are easy to control
· Normally you set night temperatures for control point (optimum temperature)
o Assume 10-15° F warmer for day
Thermoperiodicity = diurnal temperature variations.
General rule of thumb for thermoperiodicity:
· Temperatures should be higher in the day than at night
· Day temperatures generally 5-10° F higher than night temperatures
o Temperatures on cloudy days are 10-15° F lower than on sunny days
· BIG exception: higher night temperatures than day temperatures can be used to get compact, bushier bedding plants.
Effects of temperature on plant processes
· Photosynthesis - sunlight enables plants to convert CO2 and water to carbohydrates and oxygen
o When water and minerals are adequate rate of photosynthesis is determined by
§ Amount of light
§ Amount of CO2
§ Temperature
o Photosynthesis increases as temperature increases
§ To an upper limit of 95° F (35° C)
· Exception: some desert plants have a maximum temperature of 50-55° C!!
§ Lower temperature limits for photosynthesis
· Some arctic plants 32°F (0°C)
· Tropical house plants
o African Violets lower limit is 50°F (10°C)
§ Chlorophyll is destroyed below 50°F
§ Membranes solidify and leak/fall apart
§ Leaves permanently damaged if temperatures drop below freezing for even 2 minutes
· Respiration - carbohydrates broken down to release energy for growth, transpiration, movement
o Uses products of photosynthesis to manufacture food for plants
§ Ast temperature increases so does respiration
§ If respiration is higher than optimums
· Plant growth slows
· Quality is reduced
· Cost of production is increased
§ On cool, cloudy days
· Respiration is greater than photosynthesis
· Growth stops
· Need to increase light or decrease temperature
· Transpiration
o Water loss through leaves
o As temperature increases, transpiration increases (with a few exceptions)
o If the water loss is too great it will lead to wilting
o Severe water loss yields heat scorch (a permanent condition)
o Wilting is common on sunny days that follow long periods of cloudy weather
§ Leaf temperature rises 10-15° F higher than the air temperature
§ Transpiration increases
§ Water limiting
§ Plants do not recover by watering
§ Shade plants to reduce leaf temperature
· Radiational cooling
o Radiation - warm body giving off heat energy to surrounding cooler air
o The warmer the body the greater the potential loss of heat
o The cooler the air the greater the temperature gradient between plant and air
· Chilling injury can occur if move from warm air to cold too fast; the plant needs time to adapt
· On a clear, cold night after a hot sunny day
o Plants near greenhouse walls will lose heat to the walls by radiation cooling
o Temperature drop of 5°
§ Air temp 60°
· Plant temp 55°
§ Air temp 35°
· Plant temp 30° (ie frozen)
o Radiational cooling
§ May produce red pigments in leaves
· Red = anthocyanins
· Optimum temperature is 60°F, which is why Alaska has such nice red flowers!
§ Often seen in
· Geraniums
· Marigolds
· Many bedding plants
· Other temperature mediated physiological processes
o Enzyme reactions
§ Vant Hoff's law: for every 10°F rise in temperature the rate of the reaction doubles
o Amino acid and protein production
o Fat metabolish
o Flowering
§ Presence or absence of flowers
§ Flower quality
§ Flower quality
§ Flower color
§ Physiological disorders (ie bullheads in roses)
§ Chrysanthemums in temperatures that are too low (less than 60°F)
· Tubular, not flat petals
· Pink, not white petals
o Anthocyanins
§ Snapdragons in temperatures that are too low (less than 60°F)
· No petals
· Sometimes no flowers
§ Carnations in temperatures that are too low (less than 50°F)
· Red specks or streaks (anthocyanins)
· Weak, long stems
§ Ester lilies in temperatures that are too high (greater than 86°F)
· Split petals, fall apart
§ Poinsettias - flowering controlled by a combination of day length (photoperiod) and temperature
· Short day (= long nights)
· Temperatures greater than 70°F à no flower bud initiation
· Temperatures below 60°F = flower bud initiation delayed and the whole process is slowed
· Optimum temperature is 62°F to 64°F
§ Cineraria, Cypripedium, orchid stocks, geraniums
· Short day plants at 50-60°F for flower bud initiation
· If the temperature is over 60°F flower bud initiation will not occur
§ For fruit development
· Tomatoes
o Pollen is infertile if temperatures are greater than 80°F
§ Results in very poor fruit set
o Situations: poorly vented greenhouses
§ Vines near side walls
§ Vines near roof tops
o Crops grown too cool
§ Short thick stems
§ Increased time to maturity
§ Red or bronze leaves
§ Sometimes no flowers
o Crops grown too warm
§ Faded flowers
§ Long leggy stems (eg plants grown on window sills). This is due to temperature, not lack of light!
§ Reduced keeping quality (low carbohydrate reserves)
o Temperature (as well as humidity + air movement +ventilation) can influence
§ Diseases
§ Insect pests
o Temperature has a direct influence on condensation
§ Warm air holds more moisture than cold air
§ When leaves are colder than air water will condensate on them
· Best breeding ground for powdery mildew, which can kill young plants
· Powdery mildew on roses: prevention techniques
o Heat greenhouse in late afternoon
o Decease humidity (if possible) by vents
o Increase leaf temperature
o Decrease condensation
Soil temperatures
· Critical for small seedlings and rooted cuttings
· Influences movement of water and nutrients
· Influences soil microorganisms
o Greatest activity for beneficials 32° to 110° F (0 to 44°C)
o Nitrifying organisms (Rhizobia) most active at greater than 60°F (15°C) for temperate strains
· Can dictate types of fertilizer (due to microorganism activity)
o X 2 forms of Nitrogen fertilizer
§ ammonium (NH4) fertilizer: nitrogen is not readily available for plants to use
§ must be broken down by microorganisms; ammonium à nitrite à nitrate
§ nitrate (NO3)- fertilizer: nitrogen is readily available for plants to use
· In situations with cool cloudy weather; winter greenhouse; ground beds; flats put on ground
o 50% of the nitrogen should be available as nitrate or the bed/flat/etc should be heated
· Vernalization of bulbs
o Chilling period to develop roots and initiate flower buds
o Examples of bulbs that need vernalization:
§ Easter lilies
§ Tulips
§ Crocus
§ Narcissus
Handouts:
1. Standard night temperatures at which greenhouse flower crops are grown
The
current copyright laws protect this page, even though not specifically
copyrighted.
However
if you want to use portions of it feel free to do so, though I would appreciate
it if you would acknowledge my authorship.