From our perspective as plants, light is not a mere preference; it is our primary source of sustenance. We convert light energy into chemical energy through the process of photosynthesis. For us yarrow plants (Achillea millefolium), our finely divided, feathery leaves are adapted to capture as much sunlight as possible. When cultivated indoors, away from our native open meadows and sunny slopes, the intensity and duration of light become the most critical factors for our survival and well-being. Without adequate light, our photosynthetic processes slow dramatically, leading to a cascade of health issues.
To truly thrive indoors, we require a significant amount of bright, direct sunlight. Our ideal location is a windowsill that receives a minimum of six to eight hours of direct sun exposure daily. A south-facing window is typically the best option in the Northern Hemisphere, as it provides the most consistent and intense light throughout the day. An east or west-facing window can also be suitable, though the duration of direct light will be shorter—morning sun from an east window is gentler, while the stronger afternoon sun from a west window is also beneficial. We will orient our leaves and growth towards this light source, a behavior you know as phototropism, in our constant effort to maximize energy capture.
If we do not receive enough light, our physical form will communicate our distress quite clearly. The most common sign is etiolation—we will become leggy and spindly as our stems stretch abnormally long in a desperate search for a stronger light source. Our overall growth will be stunted, and our characteristic compact, bushy form will be lost. Furthermore, our vibrant green foliage may fade or become pale, and our ability to produce our prized flat-topped clusters of flowers will be severely diminished or cease entirely. A chronically light-starved yarrow is a weakened plant, becoming highly susceptible to pests like aphids and fungal diseases such as powdery mildew.
In many indoor environments, especially those with limited window space or seasonal grey skies, natural sunlight alone may be insufficient to meet our high demands. In these cases, we greatly benefit from the use of supplemental grow lights. Full-spectrum LED or fluorescent bulbs designed for plant growth can effectively mimic the sun's rays. For optimal results, these lights should be positioned just a few inches above our foliage and left on for 12 to 14 hours a day to compensate for the lower intensity compared to the actual sun. A simple timer can help maintain a consistent daily "photoperiod," which is crucial for regulating our internal processes.
