Resource: Episode Two
Want to know more about how plants respond to changing temperatures?
Check out this video.
Plants and Light
Plants use light as a source of energy for photosynthesis. The term photosynthesis refers to the reaction between carbon dioxide and water in the presence of light to produce carbohydrates and oxygen. The rate of this process is highly dependent on the light quantity.
Light is measured in Lumens (LM) which is a unit that tells you what the total amount of light emitted from a light source like a flashlight or headlamp. You can roughly say that the more Lumens the brighter the light. However, most light sensors measure lux.
Lux is used to measure the amount of light output in a given area. One lux is equal to one lumen per square meter. So if you had a 1000 lumen bulb and it was lighting up an area that was 1 square meter you would have 1000 Lux (see image to the right).
Direct sunlight is approximately 32,000 to 100,000 lux. Full-sun plants usually need at least 25,000 to 50,000 lux to do well, with increased productivity occurring when light levels are near full sunlight.
Temperature and Humidity: How to keep plants happy!
The amount of moisture in the air is generally expressed as relative humidity (RH). Relative Humidity is how much water is in the air compared to what it could hold. So 50% relatively humidity means that the air is holding half of what it could. It is important to know that relative humidity depends on temperature. This is because warm air can hold more water than cold air. This is why it is more humid in the summer time than in the winter time on most days.
Air at 70° F can hold twice as much moisture as air at 50°F. A good rule to follow is that for every 20° F rise in temperature, the water-holding capacity of the air doubles yet the relative humidity is reduced by one-half. It is very important to keep your temperature and humidity in a good range to keep your plants healthy. One of the worst things that can happen to your plants in a greenhouse is if water starts to condense on them. When water stays on plants leaves that can lead to fungus and other diseases growing on the leaves of your plants and impact their health.
Greenhouses are usually humid places due to watering and plant transpiration. This also means that if one is not careful the conditions inside of your greenhouse can be too wet for your plants. A key condition to avoid is dew point. Dew point is the temperature at which water condenses from vapor into a liquid.
As a general rule, warmer air can hold much more water than cooler air. As warm air quickly cools, as it does in Fall and Spring evenings, for example, the air also quickly loses its ability to hold water. That’s why we see dew on lawns, roofs, cars and other surfaces. The temperature at which at which the air around us reaches saturation is called the “dew point.”
Can Plants “see” light?
Plants have adapted, over millions of years, to use sunlight as their source of energy to grow through the process of photosynthesis. In photosynthesis, a plant produces sugar when a plant’s chlorophyll is exposed to light with a wavelength of in the visible part of the light spectrum of 400–700 nanometers (See the figure to the right). The amount of sugar is almost directly proportional to the intensity of the light. We can use this knowledge to expose plants to light of a specific frequency during each phase of its development.
Figure: The entire light spectrum
What type of light do plants need?
Seeds sprout in the spring. Then plants grow rapidly in the warmth of summer and produce flowers. In the autumn, we harvest fruit and other end results of this growth. How do plants know when the time is right for each step?
One of the most important factors for the health of a plant is the wavelength of light, and how long the plant is exposed to light. For plants to grow well, particularly plants like lettuce, they require both red and blue light. Flowers bloom when there is even more red light along with long hours of exposure. For a plant to produce fruit, like tomatoes, they need more yellow, orange and red light than when they were just growing (see the figure to the left).
Figure: Lights and plant growth
Why blue and red light?
It is important to note that all visible colors of the light spectrum are used by plants but blue and red are very important for plants. The reason is that the chlorophyll cells in plants are very good at absorbing and using both blue and red light.
Blue light drives the photosynthesis process and promotes good root development. If plant is receiving plenty of blue light it should have thick leaves, strong stems and will be shorter (but healthier).
Red light also helps a plant grow but is particularly important for flower production.
Figure: Chlorophyll light absorption.
Frequently Asked Questions:
What is the optimum temperature for plant's growth?
You must keep the temperature in the grow room below 80°F or it will have negative effects on your plants, especially the ones directly beneath the light. So if it gets too hot the plants will still grow, just not very fast.
How does Humidity really impact plants?
Plants need water to survive. But what happens to that water once you pour it at the base of a plant? The roots absorb the water, which then moves throughout the plant’s tissue. It eventually evaporates through the leaves and into the air. This process is known as “transpiration,” and it serves to cool a plant down so it doesn’t overheat. Transpiration also helps plants to draw nutrients from the soil, which is important for photosynthesis. (Think “sweating” for plants!)
Relative humidity affects the speed of transpiration. The higher the relative humidity, the slower the speed of transpiration, and vice versa. Any dramatic change to the speed of transpiration can damage a plant’s tissue. This is why it is important to keep conditions in your greenhouse stable over time.
My leaves are curling up. What's happening and how can I stop this?
Leaves curl up at the margin and tip in an attempt to retain water. If they are curling up, this would indicate slight environmental problem: either your light is a little too close to the plant, or your oscillating fans are drawing too much moisture from the leaves. To remedy this problem, move your light up and make sure oscillating fans are not blowing too heavily on the leaves.