The Interior Plant Environment

1. Temperature

These are guidelines for the amount of heat a plant requires to develop normally.

This assumes that all other environmental factors are also in the appropriate range.

2. Light

How Light Influences Plants

Light influences the plant's environment and its biochemical and physiological processes in many ways:

• Photosynthesis
  • Low light can inhibit growth even if all other necessities of growth are present.
• Chlorophyll synthesis
  • Green pigment in plants.
• Temperature
  • Light warms the surface of plants.

Light Wavelengths and the Color Spectrum

Light is emitted from its sources in various wavelengths.

• Light energy or wavelengths fall within 400 - 700 nanometers (NM) (PAR), which are important for plant growth.
• Visible light, the light we see, falls within PAR.
  • However, PAR extends out past the visible range into the far red, infrared, and ultraviolet.

Three Basic Light Characteristics

When it comes to light in plant growth, there are three basic characteristics to look for.

1. Quantity: intensity or brightness.
2. Duration: day length.
3. Quality: color.

A plant's response to all of these varies by the species.

Balancing plant light requirements is an important factor.

Light Intensity: Quantity

Quantity of light refers to:

• The amount of light energy the plant receives.
• Brightness of light is measured in:
  • Foot-candles (FC) or lumens.

Example: On a sunny day in the greenhouse, a crop could receive 10,000 foot-candles of light.

On a cloudy day, especially during winter, light levels are much lower.

When light intensity levels are low, supplemental light is required.

Supplemental Light for Low-Light Plants

Plants with low light requirements can be grown in areas with no natural light, but with the use of a combination of cool white and warm fluorescent tubes.

• New tubes can provide up to 1000 foot-candles when the plant is 6 - 12 inches away from the source.
• Lights are left on for 14 - 18 hours per day.

Two Ways to Look at Light Quantity

1. Illumination or photometric
   • A measurement of visible light level, as seen by the human eye.
   • Measured in foot-candles or lux/lumens.
2. Irradiation or radiometric
   • Looks at all energy from a light source.
   • Measurement is focused on the wavelength range of 400 - 700 nm.

Note: This is the region that corresponds with PAR.

Light Intensity Rating Guide

The following rating is given as a guide to the light intensity needed by a particular plant in foot-candles.

Too Much or Too Little Light

• Too much light can be as damaging as too little light.
• Plants exposed to very high light are susceptible to scalding and will not photosynthesize properly.

Minimum, Recommended, and Ideal Light

Minimum

• At this light intensity, a plant will not be able to produce energy to maintain itself over the long run.
• Over time, it will have to be replaced.

Recommended

• The level at which the plant will be able to replace old leaves as they die.

Ideal

• This intensity will provide the plant with enough light to replace old leaves and put on new growth.

Factors That Affect Light Intensity in the Interior Plantscape

In the interior plantscape, the intensity of light is dependent on a number of factors.

1. The source of light
   • How far the plant is from the light source.
2. Obstructions
   • Blinds, curtains, posts.
3. Light reflected from surfaces
   • Texture of the surface.
   • Color of the surface:
     • White reflects 90%.
     • Grey or beige reflects 50%.
     • Mirrors reflect 80 - 90%.
     • Drapes reflect 35%.

Symptoms of Incorrect Light Intensity

Symptoms of incorrect light intensity depend on the species' light requirements.

Duration: Photoperiod

Duration refers to the total measure of accumulated light over one day and is also known as photoperiod.

Houseplant lighting explained further: Growers will supplement or cut off light to control maturation.

• This is called photoperiod control.

Photoperiod Response

• Changes naturally with the season.
  • Has a definite effect on plant growth.
• The longer the photoperiod, the more time there is to photosynthesize.
  • Therefore, more energy is available for growth to take place.
  • Plants make their greatest amount of energy with long days and short nights.

Plant Signals from Changing Photoperiod

When photoperiod is reduced or lengthened, it sends signals to plants to respond in different ways.

It may induce:

• Dormancy.
• Abscission of leaves.
• Setting of buds.

Main Photoperiod Responses

1. Short-day response
   • Plants will only go into the reproductive phase under a shorter day or long night situation.
   • They will flower when they receive less than 12 hours of bright light.
   • Absolutely all light must be blocked out.
2. Long-day response
   • Plants will only go into the reproductive phase under a long day or short night situation.
   • They will only flower when they receive more than 12 hours of light.

Quality: Color

Visible light is made up of several wavelengths, each individual wavelength possessing a certain color.

• When you look at visible light from the sun, it appears white.
• If you look at it through a prism, the light is broken up into its separate wavelengths, which you can see as different colors.

The majority of light for plant growth is in the visible light spectrum.

The measurement and comparison of this color spectrum is known as quality.

• While all wavelengths within PAR are required, some wavelengths are more important than others.
• While plants are sensitive to one or more wavelengths, the human eye is sensitive to others.

Foot-Candles and Lux Lighting Terms

Foot-candles and lux measure the same thing: the amount of visible light that falls on a surface.

The difference is that the foot-candle uses the Imperial standard measuring system, such as feet and pounds, while lux uses the metric system, such as meters and grams.

A single foot-candle is equivalent to the amount of light that falls on a surface that is one foot away from a single candle, and a lux is the amount of light that falls on a surface one meter away from a candle.

For conversion: 1 foot-candle = 10.764 lux.

Additional Tips for Indoor Lighting

1. Know your plant's light requirements.
   • Low, medium, high.
2. Purchase a light meter.
   • Inexpensive option: Amazon Dr. Meter.
3. Read this article for additional ways to measure light for plants.
   • Greenery: Unlimited.
   • This will ensure the area is receiving the required light levels.

Note: If you are used to working with lux or are using an instrument that only measures lux, there is a direct conversion between the two units of measurement.

1 lux = 0.0929 foot-candle.

You can multiply your total lux by 0.0929 to get your foot-candle measurement.

3. Water

• In the interior plantscape, temperature is fairly consistent.
  • Therefore, the rate of transpiration is lower.
  • Water requirements will be lower.
  • Time of year will affect water uptake.

How Often to Water

There are no standard rules that will tell you how often interior plants will need to be watered.

How often you have to water is affected by a number of factors:

1. The physical environment:
   • Light intensity and duration.
   • Air and soil temperature.
   • Relative humidity (RH).
   • Drafts.
2. Age of plant.
3. Type of plant.
4. Size of plant.
5. Size and composition of the container.
6. Water-holding capacity of the growing medium.
7. Soluble salt content of the growing medium.
8. Plant growth activity, including time of year.
9. Condition of the root system.

Deciding When It Is Time to Water

It is important not to overwater, as this reduces the oxygen available to the roots and increases the chance of disease.

• On the other hand, never let the media dry out.
  • A plant should never be allowed to wilt.
• A plant may wilt for other reasons:
  • Overwatering, overfertilizing, and excess salts can also cause wilt.
  • Wilting can happen from low RH, bright light, and high temperatures.
  • Root rot.

Before you water, make sure watering is required.

The best way to do this is to touch the soil, look at the medium, and know how much the plant needs.

Watering Guidelines by Species

Watering guidelines depend on the species.

Moisture Meters

• Moisture meters can be added to help with determining watering needs.
• Moisture meters should not be used alone.

How Much to Water

• When watering plants in containers with drainage holes:
  • Enough water should be used to thoroughly wet the entire growing medium.
  • Allow for some drainage out the bottom of the container.
• Shallow watering will encourage roots to grow only in the top portion of the container.
  • Small amounts of water will result in a buildup of salts in the medium.

Watering Methods

Leach Irrigation

• Involves watering from the surface, allowing the water to percolate down through the medium.
• Medium should be thoroughly wetted each watering.
  • About 10% of the water should wash through the root zone into the drainage area.
  • Ensures soluble salts are kept low enough to maintain long-term root health.
• Some issues to consider:
  • Compacted and settled soil:
    • Leach irrigation may result in overwatering.
    • Root-bound plants have the tendency to be overwatered or underwatered.

Important: Never allow the plant to sit in the water that has leached through the medium during watering.

Subirrigation

• Water seeps into the media through capillary action from a reservoir at the base of the planter or planting bed.
• Allows wicks or other means to move water upward.
• Requires less day-to-day attention and provides stable soil moisture if managed properly.
• Monitor soil salts closely.
• Use good quality water.
• Replace compacted soils immediately.
• Use mulches to reduce surface evaporation.

Water Temperature

• Ideally, water temperature should be at least equal to the leaf temperature.
  • 5 C warmer will help speed up water absorption.
• Cold water cools soil temperature and slows water and nutrient absorption.
  • Cold water is also known to damage roots and leaves.
• Philodendrons and Sansevierias are examples of plants sensitive to temperature.

Water Quality

Soluble Salts

• The amount of soluble salts in the water is an important consideration when evaluating quality.
  • Water with low soluble salts can be used without any special practices.
  • Water with medium salt content is usable only if special practices are followed:
    • Provide adequate drainage in the media.
    • Do not allow the medium to become more than moderately dry.
    • Do a soil test regularly.
    • Leach periodically to remove the excess salts that will accumulate in the medium.

Hard Water

• Hard water is high in calcium and magnesium salts.
  • Gradually increases the pH of the medium.
  • If it sits on the leaves:
    • White, unsightly residue remains when it evaporates.
    • Residue is difficult to remove without a cleaning agent.

Soft Water

• High-sodium water destroys soil structure.
• Increases soil salt content.
• Becomes toxic to plants.

Chlorine and Fluoride

• Municipal levels of chlorine are not likely to cause toxicity.
  • High levels, 5 - 10 ppm, may cause minor chlorosis in some species.
• Fluoride may be injurious to some species.
  • It can cause brown, necrotic spots or lesions, especially in older leaves.

When fluoride damage is suspected, maintain higher soil moisture with aerated water or distilled water to dilute fluoride concentrations.

To minimize fluoride injury:

• Keep pH 6 - 6.5.
• Allow irrigation water to sit open at room temperature for 24-hours.
• Avoid superphosphates or fluoride-containing amendments.
  • Perlite in the medium.

4. Relative Humidity

Relative humidity (RH) is the actual humidity of the air.

• The warmer the air temperature:
  • The more water it can hold.
  • RH decreases.
• In Alberta in the winter, RH can be as low as 15%.
• Some plants require 60% or more.
• Most indoor plants are produced in an environment with much more moisture in the air than is found in the average home or building.

Ways to Increase Relative Humidity

• Build a humidifier in with the heating system.
• Install a misting system.
• Group plants so moisture from transpiration creates a microclimate.
• Use a tray filled with gravel or sand, sit plants on top, and fill with water.
• Use a terrarium.
• Place plants in areas where there is naturally more humidity, such as a bathroom.

Note: Misting needs to be done regularly to keep humidity up.

• May cause disease problems.
• Increase air circulation to help the leaf surface dry quickly.
• Mist in the mornings.

5. Media

Whether you buy premixed media or mix your own, the ultimate goal is that the media fits the required criteria.

Indoor Plant Soil Requirements

Indoor plant soil requires these properties:

1. Support: physically supports the plants.
2. Water-holding capacity.
3. Air spaces.
4. Nutrient-holding capacity.
5. Stability.
   • Plants will be in the mix for a long time.

Potting Mixes

Different potting mixes are on the market, including specialty mixes like:

• Cactus mix and azalea mix.
• Often, mixes are peat-based media enhanced with:
  • Perlite.
  • Vermiculite.
  • Sand.
  • Composts.
  • Fertilizers.
  • Water crystals.