Why light levels are important for indoor plant growth

plants growing under artificial light indoors
Photo: Miri Talabac

Typically, insufficient lighting is the limiting factor for indoor plant growth and flowering, plus the reason for spindly seedlings. Most light fixtures in our homes and offices – especially those still using incandescent bulbs or those with energy-saving bulbs that mimic incandescents – don’t give off enough energy for plants to survive or thrive on long-term. In addition, windows block a surprising amount of sunlight intensity compared with the same spot just outside the glass. Insect screening outside a window reduces intensity even more.

Artificial lighting can either supplement natural light or be the sole light source for plants. Plants tolerate levels of light outside of their preferred range, but sensitivities vary from species to species. Over time, the consequences of inappropriate light levels may impact a plant’s health and alter its appearance, even if it isn’t immediately noticeable.

The importance of light to plants

For plants, light is food. We think of fertilizer as plant “food,” but in reality, it is more akin to a multivitamin than it is a meal – it supports how they use their food (carbohydrates from photosynthesis) and helps build tissues, pigments, hormones, defensive chemicals, and so forth – but it’s not providing the calories they need to survive and grow. They certainly can “fast,” so to speak, such as spending a few days in a box in transit or remaining semi-dormant in winter, but prolonged light deprivation from insufficient lighting will have negative impacts on plant health akin to slow starvation.

Plants may lack eyes, but they can still “see” light by detecting its colors, intensity, and duration. Coupled with temperature or precipitation, it can tell them what season it is for the purposes of growth and reproduction (flowering). Weather can fluctuate from year to year, but the patterns of daylength and general light intensity remain the same and are the most reliable environmental cues for the plant.

Plant reactions to light

As plants grow, their leaves develop in response to the environmental conditions they are experiencing at that time. This allows them to photosynthesize as efficiently as possible. However, this means they have limited ability to change their leaf physiology after the leaf has matured in order to deal with markedly different levels of light. This is one reason plants adjusting to a change in conditions (such as going from a greenhouse or patio to inside your home) may drop leaves or need to replace leaves in order to acclimate.

indoor plants growing under artificial light
Photo: Miri Talabac

How plants deal with too much or too little light

Plants cannot move very far when seeking more or less light. Instead, they adjust their physiology to try to adapt. While fully-developed and expanded leaves don’t change size, a plant can change a leaf’s position by tilting it to intercept more light or to avoid bright light. Plants can also change stem length as they grow, reaching or leaning towards light if they’re getting too little, or staying squat and somewhat stunted if getting too much.

The points on the stem where leaves attach are called nodes. The sections of stem between the nodes are called internodes. In bright light, internodes tend to be short so the leaves are held closer together, where they can shade each other. In dimmer light, internodes tend to be long, so each leaf collects more light and self-shading is minimized. Etiolation is the term for longer-than-usual internodes, when a plant is “stretched” or “leggy” and trying to get more light.

Other changes can occur as a plant reacts to increases or decreases in light while it’s growing.

  • In light that’s too bright:
    • new leaves mature smaller than average because they don’t need as much space to collect enough light
    • foliage that is normally deep green can “bleach” or “sunburn/scorch” as intense light damages tissues, or it can blush reddish as a means of protecting itself with pigments that act like sunscreen (sometimes referred to as “sun-stressed”)
  • In light that’s too dim:
    • new leaves mature larger than average because they need more space to collect enough light
    • foliage with rich colors – burgundy, red, yellow – become greener when light isn’t intense enough because the plant needs to use more chlorophyll
    • variegated plants may lose some of their multi-colored patterns in insufficient light
    • growth slows or becomes sparser overall and plants may refuse to bloom
intense light changes leaf color from green to reddish
Photo: Miri Talabac

What are your plants telling you?

If you’re already growing plants indoors, you may have noticed changes in their appearance, either since you brought them home or over the course of a year. Some of these changes may be due to differences in lighting, though other factors can play a role as well and complicate your evaluation. (Soil moisture, nutrition, humidity, temperature, and plant stage of maturity can all influence leaf appearance and plant growth.) If you think a particular plant has been declining or failing to thrive, see if key features – leaf color and size, stem length – suggest light levels are the reason.

Coming up: we’ll dive into some jargon! In our next installment, we’ll introduce indoor lighting options and some technical terms that are important to understanding what product specifications mean. Don’t be intimidated or skip over this – it’s very useful in helping you decide how suitable a light will be to give you the best performance.

By Miri Talabac, Horticulturist, University of Maryland Extension Home & Garden Information Center. Read the previous article in this series, An introduction to gardening under lights, and additional posts by Miri.

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