How changing sun angle affects fall blooming perennials - Ask Extension
I’ve been in my new (to me) one story house for a year. It has 8-10 ft beds along the front of the which I’ve prepared as a clean slate for new la...
Knowledgebase
How changing sun angle affects fall blooming perennials #947552
Asked October 23, 2026, 10:12 AM EDT
I’ve been in my new (to me) one story house for a year. It has 8-10 ft beds along the front of the which I’ve prepared as a clean slate for new landscaping. The house faces north north east and gets full day sun for all of summer, but by fall, it only gets a couple of hours of sun. I’m wondering if the all day sun in summer will be enough to develop flower buds on late blooming perennials (asters, solidago) so that they will still bloom even though they are part sun at best by the time of flowering. Or do they need full sun through their blooming? Thanks
Montgomery County Maryland
Expert Response
The main season for energy-storing photosynthesis and growth is summer (probably starting in late spring too), when the sun's light intensity and duration is at its greatest. Fall-blooming plants are evolved to deal with lower light levels and shorter daylengths by the time they flower; by then, they've already stored enough energy to bloom or they would not be flowering (regardless of how much sun they're exposed to in autumn). When the light exposure recommendations (full sun, part sun, etc.) for plants is given in its care information, it's referring to the peak of sunlight in summer.
As an aside, a northern/eastern exposure against a building typically does not get full sun in summer since the sun is angled slightly to the south while overhead, so the southern and western sides are usually the sunniest, excluding any nearby tree canopies. Usually, the building itself casts some shade for part of the day on its northern and eastern sides, especially after midday. "Full sun" is considered to be a minimum of 6 hours of direct sun (in summer), although 8 to 10 hours is often ideal for sun-loving species. Since the sun's light is more intense around midday and early afternoon, a full sun exposure tends to include some midday and afternoon hours, even if a site is very sunny in the morning but in semi-shade or shade by early afternoon.
For example, sites with the same 6 hours of sun duration can differ in their impact on plants due to how many of those hours are providing more intense light. A location with direct sun from about 7am to 1pm before something (a building or a tree) casts a shadow on the area may not support as much vigor and blooming for a sun-loving plant than a site with direct sun from about 10am to 5pm. Most plants are flexible enough that this difference won't matter too much, but in that example you'd probably have the plants in the latter site flowering earlier and longer than those in the former site because, overall, they're getting more light due to more of those hours being at the greater light intensity.
Miri
As an aside, a northern/eastern exposure against a building typically does not get full sun in summer since the sun is angled slightly to the south while overhead, so the southern and western sides are usually the sunniest, excluding any nearby tree canopies. Usually, the building itself casts some shade for part of the day on its northern and eastern sides, especially after midday. "Full sun" is considered to be a minimum of 6 hours of direct sun (in summer), although 8 to 10 hours is often ideal for sun-loving species. Since the sun's light is more intense around midday and early afternoon, a full sun exposure tends to include some midday and afternoon hours, even if a site is very sunny in the morning but in semi-shade or shade by early afternoon.
For example, sites with the same 6 hours of sun duration can differ in their impact on plants due to how many of those hours are providing more intense light. A location with direct sun from about 7am to 1pm before something (a building or a tree) casts a shadow on the area may not support as much vigor and blooming for a sun-loving plant than a site with direct sun from about 10am to 5pm. Most plants are flexible enough that this difference won't matter too much, but in that example you'd probably have the plants in the latter site flowering earlier and longer than those in the former site because, overall, they're getting more light due to more of those hours being at the greater light intensity.
Miri
