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Tuesday - September 29, 2009

From: Phoenix, AZ
Region: Southwest
Topic: Best of Smarty, General Botany
Title: Disappearing sunlight in Phoenix, AZ
Answered by: Barbara Medford


I live in a condo in Phoenix, AZ with a north facing patio that goes out about 10 feet and is 20 feet wide. During the summer months there is a span of 1 foot in the front that goes the 20 foot length where there is full sun. This summer I got some containers and planted blackfoot daisy, bougainvillea, rosemary and some herbs in that patch of full sun. This past week I bought some Fragrant Cloud and Diana, Princess of Wales (re-named Elegant Lady) hybrid tea rose trees to put in this area as well. I got the roses planted in their containers and looked the next day and was surprised to realize there must have been a shift in the tilt of the Earth because I now get NO SUN!!! The shade goes until about 2 feet past my fence into the walkway of the condos. Do you have any suggestions for what I can do with my plants and if they will survive this fall and winter in the full shade? I know UV rays are still outside regardless of if it is shade or full sun, but I am not sure how it works for plants. Any help offered would be extremely appreciated.


Just for openers, we need to tell you that the expertise and research at the Lady Bird Wildflower Center is all about plants native not only to North America but to the area in which they are being grown. Of all the plants you listed, only Melampodium leucanthum (plains blackfoot) is native to North America, and it is not native to Arizona.  But we can tell you from experience and observation that all these plants require full sun (6 or more hours of sun a day) and some can tolerate part sun (2 to 6 hours a day).

You say that your sunlight disappeared, but it didn't do that overnight. And you thought the Earth had tilted, but it is already tilted, at about 23 degrees. Because of this tilt, in the summer the Northern Hemisphere is tilted more toward the sun; the sun rises higher in the sky and is above the horizon longer. In the winter, the Northern Hemisphere is oriented away from the sun; the sun rises lower in the sky, is above the horizon for a shorter period each day, and the sun strikes the ground more obliquely.

If plants have nothing near them, as in a building, and nothing above them, like a tree, they will get whatever sun there is, regardless of the season.  But, visualize your patio: In the summer, the sun is overhead, the building is not casting much shadow, and your strip of sunshine is there. But, now, as winter approaches, the sun is dropped down in the south. When you're facing north, the building is between you and the sun, and you are in the shade, and so are your plants. As you go farther north, this situation is exaggerated beyond what you have in Phoenix; the days are shorter, the nights are longer, and there is less sunlight all around. Plants adapt to this by seeding and dying, if they are annuals, dying to the ground to grow back from the roots when there is more sun if they are perennials, or by becoming semi-dormant, not needing so much sunshine. 

You mentioned UV rays being outside regardless, but that is not how plants use the sunlight. When sunlight strikes a leaf, a process called photosynthesis is put into play, the plant converts the energy from the sun, combines it with water and nutrients in the plant, and metabolizes it into food to support the plant, form new structures within the plant, and store food in the roots. Along the way, it releases oxygen, which is a good thing for the human race. That is a very much simplified explanation, but we wanted to make sure you understood why the plants needed actual sunlight. 

So, to get back to your question. During the period of rapid growth, blooming, setting seed and so forth, those plants will probably have about 4 or 5 months of sunlight during the year. They might even survive with the periods of no sunlight, but they will not flourish. They could be stunted, they probably won't bloom much, and won't be as attractive as you hoped. Phoenix, in Maricopa County, is in USDA Hardiness Zones 9a to 9b, where the average annual minimum temperature is from 20 to 30 deg. F. Probably all those plants will be all right in pots in those temperatures, as it's very doubtful that the temperatures would ever drop low enough for the roots to freeze. Whether you want to continue with those plants out there is up to you, you can't change the amount of sunlight they are going to get. 


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