Plant Health Trilogy Part 3: Light

For plants to thrive, their needs need to be met with regard to three critical elements: Water, Soil and Light. In the last two newsletters we talked about the importance of proper watering and appropriate soil as they impact the health of your plants. 

The final part of the trilogy is LIGHT, and this is a deceptively challenging element because sun exposure changes throughout the year depending upon the angle of the sun and the leaf canopy of deciduous trees.

Many spring ephemeral plants use the sun that comes through the deciduous tree canopy to flower and disappear as the leaves break and the heat starts. I plant Dahlias on a side of my house that gets no sun in the Winter but gets 6-8 hours from April through September, and they get more as they are 4′ tall in the fall.

The point is that the sun exposure often changes throughout the seasons and overstory trees can grow and limit sunlight over the years.  I go through this detail because everyone, including me, will sometimes over-estimate the amount of sun that a certain location gets, and with plants that need Full Sun, less than six solid hours will effect their health, flowering, size and vigor.  Who hasn’t put a rose in a bed that is smoking hot and sunny at one in the afternoon but come to realize that it only gets five hours of sun in June and July.

If watering properly is feel and experience, and soil management is science, then sun exposure is honesty. You need to be honest in your assessment of a location and not over or underestimate sun exposure.

• Full Sun – 6-8+ hours of sun during the growing season, especially the peak day sun.
• Part Sun – 4-6 hours of sun
• Part Shade – 3-6 hours of sun, without peak day sun.
• Shade – less that 3 hours of sun, especially without peak day sun.

I use an app called Sun Tracker, to look at the exposure in a specific bed.  The 3D view allows me to use my phone’s camera and literally track the sun’s path across the sky during different seasons and see how tees and structures interfere with the sun. A very handy app.

When plants are labeled for full or part sun, it is critical that they get the minimum amount of sun exposure, any less will start to impact the health and performance, but with part shade and shade loving plants, they can often do well with more sun, but they may need additional water to remain healthy.

As I mentioned earlier, peak day sun from 11am-2pm is critical for sun-loving plants with its heat and intensity.  It is hard to give these plants what they need if they are shaded during this period.  Shade or part-shade plants will resent this direct midday sun, but will do better if the sun is dappled by high tree cover. While proper sun exposure helps plants to flower best, not enough will also contribute to fewer flowers, weaker growth, and less vigor.

There is also a difference between morning and afternoon sun. 5-7pm sun is more intense and drying than 7-9am sun. Macrophylla Hydrangeas (blue/pink ones) will do really well with early day sun before the heat builds too much, and they can recover better in the heat if they are out of the sun in the afternoon. But you will notice continual leaf flopping if they get all their sun in the afternoon.  The plant just doesn’t have the ability to replace all the transpired water from the leaves in hot afternoon sun. They require part-sun to part shade exposure, but you need to find the line of getting enough sun to flower well but no too much that it stresses the plant. In a more sunny location it will require large amounts of water to keep it from getting stressed and burned.

Many shade loving plants like Hosta, Fern, Solomons Seal and the like really don’t like a lot of hot, direct sun, but with increased watering will tolerate it, with some species doing better than others.  Pay attention to plant labels or do your own research, there are several Hosta that actually prefer more sun, so there can be great variation within a genus of plant. 

You really need to be honest in matching a plant’s needs with its location.  Use the Sun Tracker App I mentioned to get a clear idea of sun exposure in your bed, knowing that one end of the bed could have an hour more/less of sun.

I hope that these three installments discussing Water, Soil and Light were informative and give you a greater understanding of how to make your plants thrive. If you apply what you have learned, you can triage any plant challenges and figure out a solution. Subtle changes in light, soil or water can make a big difference in the performance of your plants.