Our yuccas have been planted for over four years now, maturing very well and reaching heights about 2 meters. They have a very large trunk, which produces new yuccas all the time. I then let the new growth grow to about 1 meter before using them in other parts of the garden, which has worked well. My concern is the trunk is very big. How big are the roots, as we have it in a garden bed with a retaining wall? Also, should I cut back the mature yucca?
Whether you cut back the mature yucca is up to you. They do not need it, but it will not hurt them.
In terms of the retaining wall, there is a chance that the roots could damage the wall in future. It really just depends on how much space the roots have. Walls and foundations are no different than pots to a plant. If the plant runs out of room for the roots to grow, it will push against the walls of its containment, be it a pot or a wall, and try to make room. You may want to dig a small hole on the yucca side of the wall once a year or so to check if the roots have reached it yet. If they have, you can try root pruning the plant to control the roots.
We have a very large yucca plant in our back garden with three main trunks on it. Each trunk has an additional two newer and smaller trunks growing from it. The plant has been in the garden over ten years. After the harsh weather and heavy amounts of snow, the leaves have virtually all fallen off and the top part of the trunk where the leaves are growing from has rotten. Some are black and some are just so soft and squisgy. We will be devastated if the plant cannot be saved, as it is the main feature of our garden. Can it be cut back and will it regrow? I am not a gardener by any stretch, so any help will be appreciated.
I would wait until spring to cut back the plant. It will help protect the new growth, just in case another freeze occurs. These articles will help you as well:
The pruning instructions here apply to outdoor yucca as well:
Outside Yucca got badly damaged in the heavy snow, leaves are brown and flopping around the stem.
I am sorry to hear that your plant was damaged. This article will hopefully help:
How do I know how much water and sunlight to give to a Yucca plant that is about four feet tall?
Most yuccas need bright light for several hours a day. Water the plants when the soil is dry to the touch.
I have a 7-trunked yucca about 3. 5 metres tall. It has flowered a couple of times and is starting to look a bit thin on top. Should I do something or just let it carry on?
You can try topping a few of the trunks to encourage thicker growth. This involves cutting off the top 1/3 of the branch (which you can root as a new plant).
Will my yucca recover from severe frost damage?
This article may help: https://www.gardeningknowhow.com/ornamental/foliage/yucca/yucca-cold-weather.htm
We have two yucca plants outside our window growing together. How do I know what to prune? Do I remove all the brown leaves, and what's the best way to separate the two plants?
This article should help you with that: https://www.gardeningknowhow.com/ornamental/foliage/yucca/yucca-plants-care-and-pruning-tips-for-pruning-a-yucca.htm