How to kill a plant: death by light

This is my windowsill.

This is my windowsill.

When people ask me, “What kind of plant should I get?” I first reply with, “In what direction do your windows face?” 

Every windowsill is a unique space that provides a certain environment to plants. Knowing the amount of light a plant might receive in the environment is important for choosing the right plant. Not all plants require the same light amounts or conditions, but all plants do require light since it is the key driver of growth. Keeping a plant in the wrong light environment will first lead to poor growth and ultimately leads to death. (This method of killing a plant is one I have personally failed at - so far.)

First, lets think about what happens when light hits a plant leaf. 

Cell types in a leaf. Image from wikipedia

Cell types in a leaf. Image from wikipedia

Leaves are structures that specialize in capturing light. Cells on the leaf surface, called the epidermal layer, are concave shaped to catch as much light as possible. Below these cells, one to three layers of pillar-like palisade cells contain chlorophyll and photosynthetic action happens here. Cells below these in the spongy mesophyll have large air and water spaces surrounding them. Once light reaches these cells it is reflected and ‘scattered’ by the water. 

When solar energy hits a plant, chlorophyll pigments absorb red and blue wavelengths. This energy is used to ultimately create carbohydrates. Wavelengths in the green part of the spectrum are reflected, which is why most plant leaves are green. 

Plants contain other pigment molecules besides chlorophyll, which reflect different colored wavelengths. This is why some plants have leaves that aren’t green. These pigments Carotenoids reflect yellow to orange wavelengths, and assist chlorophyll in photosynthesis. In times of too much light these pigments move light away from the electron transport chain (the beginning of photosynthesis) and dissipate the energy as heat. Anthocyanins are another type of pigment molecules in leaves and stems. During times of high-light stress they act as plant ‘sunscreen’ by absorbing green and blue wavelengths and reflecting red to purple.  

Now, think about where houseplants originate.

Most house plants come from exotic far away places. Cacti and succulents come from the desert, where water is scarce, soils are predominately sandy and light levels are high. Philodendrons and pepperomia (two of my favorite houseplants in the Northeast US) are tropical or subtropical understory plants. They originate from a warm, humid environment that would only ever receive dappled indirect sunlight. Plants from these two groups both grow in my apartment because I try to mimic their native environment as much as possible.  Monsieur le Jade plant lives on a bright, south-facing windowsill. Mme. Pepperomia resides on the kitchen counter where direct sun won’t hurt her complexion.

 
 

What happens if a plant doesn’t receive the right amount of light?

Plants have mechanisms too cope with too much or too little light. Leaf movement is one. Windowsill dwellers only receive light from one direction. To collect as much light as possible plants will bench towards the light source and start growing in that direction. If this happens for a long enough period of time, it can permanently change the shape of the plant. Some houseplants, those with broad leaves, will tilt their leaves towards the light-filled window as well.

Too much light exposure can damage the photosynthetic process within a plant, and bleach the leaves. Eventually this will kill the plant. To combat this plants in excess light may move their leaves perpendicular to the light source to have less exposure. Wilting can facilitate this! On a hot sunny day, a droopy, wilty leaf can be a protective measure because less surface area is exposed to the sun. (Although, if your houseplants are drooping it’s probably because you didn’t water them enough.) 

"Chloroplast Movements in Response to Light. Chloroplasts within mesophyll cells of duckweed (Lemna) can shift their position within the cell to absorb more (I, II) or less (III) light. This mobility of chloroplasts helps to enhance photosynthesis at low light levels and avoid damage at high light levels."  Figure from 'Ecology' by Cain, Bowman and Hacker

"Chloroplast Movements in Response to Light. Chloroplasts within mesophyll cells of duckweed (Lemna) can shift their position within the cell to absorb more (I, II) or less (III) light. This mobility of chloroplasts helps to enhance photosynthesis at low light levels and avoid damage at high light levels."  Figure from 'Ecology' by Cain, Bowman and Hacker

Plants do a similar thing at the cellular level. In periods of low to optimum light chloroplasts line up along the surface of the cell wall that is parallel to the sun. If there is too much light they line up along the call wall perpendicular to the sun so less light is absorbed.  

Older leaves on the bottom and right are yellow and unhealthy compared to new green leaves.

Older leaves on the bottom and right are yellow and unhealthy compared to new green leaves.

I said I have yet to kill a plant by keeping it in the wrong light environment, but that doesn’t mean I haven’t come close. Take for example the case of the begonia - a common houseplant (and outdoor annual in the garden). I put this shady, indirect light loving plant on a sunny windowsill throughout the month of February. As the days got longer nearby plants grew like crazy, but not the begonia. Leaf color changed from dark and glossy to pale and sickly.

Do you have an unhappy looking plant? Is it because it’s not receiving the right amount of sunlight? Or is it due to some other stressor? 

There are lots of ways to kill a plant: