What causes total defoliation on boxwood shrubs?
It is possible your shrub is affected by boxwood blight. Here is more information: httpss://yancey.ces.ncsu.edu/wp-content/uploads/2013/02/Boxwood-Blight-Guide-01.03.13.pdf
If have some boxwoods around 28 years old that are turning yellow and some newer ones around 3 years old. What can I do to save them? Why are they turning yellow? Do boxwoods have a certain life span?
Since your 3 year old boxwoods are yellowing too, it is not an issue of aging or life span. There is something else amiss. And, I have read about boxwoods that are 40-50 years old so your boxwoods are young compared to that!
There are many reasons for boxwoods turning yellow which include winter injury, root rot, nematodes, macrophoma leaf spot, and volutella blight.
The following article will help you pinpoint the cause and provide a course of treatment:
https://www.gardeningknowhow.com/ornamental/shrubs/boxwood/yellow-or-brown-boxwoods.htm
Boxwoods suffered winter kill. Somewhat extensive. How can I try to renew them? They are approx 15 years old, 3 feet high, 3 feet in diam.
When you say the winter kill damage was extensive do you mean that the damage extends beyond the foliage (for example - is the bark splitting?) And are only the tips of the boxwood impacted - winter burn commonly just yellows the tips of boxwoods but can be deeper and take over a whole bush in more severe cases.
The best thing you can do to help your boxwoods recover is to prune the dead and damaged parts of the plant. You can cut back the tips of the damaged branches to just above a green living leaf. - it is okay to do major trimming in the early spring once the threat of extreme cold snaps has passed and before the boxwoods sprout new growth.
For more information on how to protect your boxwood from future winter injury, please read the following link:
https://www.gardeningknowhow.com/ornamental/shrubs/boxwood/boxwood-winter-protection.htm
For more information on pruning boxwoods, please visit the following link:
https://www.gardeningknowhow.com/ornamental/shrubs/boxwood/trimming-boxwood-bushes.htm
We have rows of Boxwood hedges, which often turn yellow, and need to know what is the best food to use. We were told that they don't like acidic soil.
Here is a link that will help.
https://www.gardeningknowhow.com/ornamental/shrubs/boxwood/fertilizing-boxwoods.htm
I have a very old (30 years at least) boxwood hedge, which is spindly and unattractive. Can it be pruned and, if so, how or do I have to take it out?
I would recommend that your boxwood shrubs undergo what is called a "renewal pruning" which is a multi-year pruning process which involves pruning it back in stages. Renewal pruning is described in detail here: https://extension.psu.edu/plants/green-industry/news/2012/hacking-whacking-dealing-with-overgrown-shrubs
For more information on the care of boxwoods, please visit the following links:
https://www.gardeningknowhow.com/ornamental/shrubs/boxwood/boxwood-care.htm
https://www.gardeningknowhow.com/ornamental/shrubs/boxwood/trimming-boxwood-bushes.htm
What causes a boxwood to be rigid in morning but look wilted in the evening? I have 4 boxwoods that look wilted at the late afternoon where the sun has been out most of the day.
I would say what you are seeing is normal for a boxwood exposed to hot afternoon sun. And, your shrub still looks healthy - I see no yellowing or browning or any sign that it is too distressed.
For more information on boxwood trees, please visit the following link:
https://www.gardeningknowhow.com/ornamental/shrubs/boxwood/boxwood-care.htm
I'm very concerned about the health of my boxwoods. Any guidance would be greatly appreciated! For many years my boxwoods thrived; however, I fear I may have trimmed them at the wrong time of year in combination with a very tough winter. Currently, many of the outermost branches have very few leaves as well as a yellow tint. I'm hoping that water and proper fertilizer could help revive them. Thank you so much!
You have a beautiful stand of Boxwood!
Yes, I do think your hedge has suffered some winter damage.
I would allow the plants to recover with adequate water.
Here are some great links to help you with recovery and proper pruning.
https://www.gardeningknowhow.com/ornamental/shrubs/boxwood/boxwood-care.htm
https://www.gardeningknowhow.com/ornamental/shrubs/boxwood/trimming-boxwood-bushes.htm
https://www.gardeningknowhow.com/ornamental/shrubs/boxwood/yellow-or-brown-boxwoods.htm