Alison Arnold: Solar Eclipse And Ways Light Affects Plants

Aug 10, 2017

This week, BPR gardening expert Alison Arnold talks about the solar eclipse and the ways light affects plants.

Q: Recently we talked about doing a sun study and I began thinking about light and it’s effects on plants.. with the upcoming solar eclipse coming is there any cause or effect it will have on plants?

A: The little research I've been able to do indicates there have been studies that show a Solar eclipse does have a small affect on the process of photosynthesis since a solar eclipse blocks out some sunlight, the rate of photosynthesis will be slower than usual.

Q: Photosynthesis is the process where plants manufacture their own food – correct?


A: Yes, so plants use the energy from sunlight, carbon dioxide from air, and water from the soil to produce simple carbohydrates. These carbohydrates are converted into more complex sugars that can be transported to the stems, roots, flowers, and developing tissues for immediate use or they can stored or used as building blocks for other structures, such as cell walls. So if the amount of light is reduced this entire process can slow down. Probably not detrimental in this case given the short duration of the eclipse.

Q: Are there other ways light affects plants?

A: Phototropism for instance is the plant's movement in response to light. We’ve all seen plants that lean toward a window or area of greater light. This is called phototropism. Growth hormones are involved that stimulate cell growth of the stems to cause the plants to grow in the direction of the light, allowing leaves to be closer and intercept the more light for better photosynthesis.  And there’s another response called photoperiodism that controls things like seed germination, dormancy, and flowering in day length sensitive plants.

Q: Can you say more about the seed germination and dormancy?

A: Well … Some seeds are light sensitive such as many lettuce varieties and they need light in order to germinate. Many weed seeds are also in this category which is why weeds can come on strong after tilling or disturbing the soil, they lie dormant in the soil waiting for some disturbance so they can get enough light to germinate. Collectively these seeds in the soil are often referred to as a seed bank which is why no till techniques are helpful in keeping weed problems down in the garden.

Q: I have one last question then about light in the garden…. is there a difference between part sun or part shade?

Unfortunately, this question is open to interpretation and there are no clear definitions… however most sources agree that full sun is 6 or more hours of direct sunlight per day. It doesn't have to be continuous as long as it’s direct full sun. Most references say that part sun / part shade is 4-6 hours of sun in a day with the part sun being at the upper level and part shade at the lower level and full shade is simply the lack of sunlight or anything less than 4 hours of sunlight a day…