Ask a Question forum: ??? Regarding sunlight

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NE Pennsylvania
Sep 26, 2017 12:41 PM CST

****Is it the sun's RAYS (brightness) that hurt the plant or is it the HEAT?****
**** Is full outside unshaded sun in Sept/Oct as bad as full direct unshaded sun in July /Aug****

Would putting a plant in the full bright day sun on say a 60-65 F degree day basically be the same as doing it on a 90-95 degree day? I am in NE Pennsylvania and fall is coming but chilliness is not here yet. I have already noted the "angle" of the sun has/is changing and I would like my plants to enjoy actual sunlight as long as possible before they come in for winter. Thanks
[Last edited by JungleJen - Sep 26, 2017 1:00 PM (+)]
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Name: Baja
Baja California (Zone 11b)
Cactus and Succulents Seed Starter Foliage Fan Xeriscape Container Gardener Hummingbirder
Native Plants and Wildflowers Garden Photography Region: Mexico Plant Identifier Forum moderator Plant Database Moderator
Sep 26, 2017 12:57 PM CST
The sunlight itself is potentially dangerous because of both intensity (direct overhead rays are the strongest) and duration (days are longer, with more hours of sun) in summer. A combination of the two is particularly bad. If you want to provide a lower-intensity situation for the accommodation process, the easiest solution is usually overhead protection (like right by your house under the eaves, or on a covered porch) which would block the overhead rays but not the more sideways ones, which are weaker.

If you are trying to choose between morning and afternoon sun in a situation like that, choose the morning sun (E exposure) because sun combined with the heat of the afternoon is more dangerous than sun in the cool of the morning. Intense sun is much less dangerous here in our coastal zone, where temps are generally quite mild, than it would be in the hotter interior. Temperature definitely helps determine how dangerous sun can be.

The combination of sun and heat tends to have its most potent effects on the roots of potted plants, actually... a dark pot sitting in the sun will get quite hot to the touch and the roots inside can end up baked like a casserole. You will earn valuable protection by nesting or grouping your containers so that they are not all exposed to the sun at the level of the container. And you can put your hand on the pot to get some idea of how hot it's getting.

The greatest intensity of sun comes around the start of summer, which is when days are also their longest. The intensity is related to how close the rays are to vertical: the more overhead the stronger they are. This time of year they are starting to hit more from the side and should be kinder to your plants, barring any kind of intense heat. I generally like to move my nursery plants into the sun (after some gradual accommodation) this time of year actually, because the danger is less.

The difference between indoor sun and outdoor sun, all things equal, has to do with the fact that regular window glass blocks much of the harmful UV rays. The hardest thing up front for indoor plants going outside is probably going to be that adjustment to the UV.

[Last edited by Baja_Costero - Sep 26, 2017 2:41 PM (+)]
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Name: Will Creed
Professional indoor plant consultan
Sep 26, 2017 4:54 PM CST
If your plants have done well outside during the summer, there is no reason to relocate them now because the light is changing. They can adapt to that gradual change. To a certain extent, they can also adapt to gradually cooler temps. However, if some of your plants are tropical or semi-tropical, a sudden cold snap could cause serious damage. Know what the low-temperature tolerances are for your plants and monitor the temperature forecast carefully so you get them inside soon enough to avoid chilling damage.
Will Creed
Horticultural Help, NYC
Contact me directly at [email protected]
I now have a book available on indoor plant care

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