I have had this plant since it was tiny and it has been healthy and flowered every year with many bloom clusters which the bees love. This year the Bark is falling off all over the trunk as seen in photos. There is no other obvious sign of disease or insect. The bark seems to be lifting off then dropping randomly all around the tree trunk which is about 15 - 20 feet tall.
It might Have Armillaria or other similar root rots! Treating with a fungicide, right away, will be the best option.
https://www.gardeningknowhow.com/plant-problems/disease/armillaria-root-rot-control.htm
https://www.gardeningknowhow.com/plant-problems/disease/root-rot-in-garden-plants.htm
I have recently had 4 yukkas removed and the stumps ground to chips. Is it OK to leave these in the soil for drainage? They were poisoned by drilling holes in the centre 4 months ago, so would there be any residual poison still in the chips? I do intend to add compost and manure to the soil as well before planting anything else. Thank you
It depends on what you used, but some stay in the soil for around 6-8 months. As long as you leave it for that time, then add organic material, it should be fine.
These articles will help you to keep your soil tested:
https://www.gardeningknowhow.com/garden-how-to/soil-fertilizers/testing-moisture-in-plants.htm
https://www.gardeningknowhow.com/garden-how-to/soil-fertilizers/testing-soil.htm
https://www.gardeningknowhow.com/garden-how-to/soil-fertilizers/soil-ph-plants.htm
https://www.gardeningknowhow.com/garden-how-to/soil-fertilizers/test-soil-for-pests-and-diseases.htm
https://www.gardeningknowhow.com/garden-how-to/soil-fertilizers/making-sure-soil-drains-well.htm
What should I use to fertilize my outdoor Yucca plants? How often and when is the best time of the year to do it?
Because this is a desert species, they do not require much fertilization. If this is grown in a decent quality soil that is not compacted, then fertilization will only be necessary when a soil test reveals nutrient content to be low. You will likely never see this, but if you do, an all purpose at half strength will do.
https://www.gardeningknowhow.com/garden-how-to/soil-fertilizers/testing-soil.htm
https://www.gardeningknowhow.com/garden-how-to/soil-fertilizers/soil-ph-plants.htm
I had a large Yukka in my garden which after wet dec last year followedd by frost killed the healthy tops off. Was 3.5 mtr high have cut down to 1 mtr but has not sprouted. How can I encourage growth? I did seal the to after cutting back. Please help!
Generally, sealing is considered an out of date practice. It tends to seal moisture to the cut area, which can make diseases worse. It was once thought to prevent it.
https://www.gardeningknowhow.com/ornamental/trees/tgen/should-you-seal-a-tree-wound.htm
However, you can cut it down further to allow it to "harden off".
If and when the base sprouts pups, you can transplant them, or leave them to form a sort of "shrub":
My Yucca is 9 years old and flowered for the first time last year, but this year it has three spires, see attached photo, please advise if it requires any additional attention, as I am told that three flowering spires is very unusual?
It happens, sometimes. There isn't much to do, other than letting it continue on. Once it does finish, you can cut them as far down into the plant as possible to remove them. This article will help you know what is next after flowering:
https://www.gardeningknowhow.com/ornamental/foliage/yucca/yucca-care-after-blooming.htm
I have many yucca’s seeds. How do I use these?
You can grow more plants! Here is how:
https://www.gardeningknowhow.com/ornamental/foliage/yucca/yucca-seed-pods.htm
My plant has grown out of control in my front garden. I would like to reduce the height and the width of it. What is the best way of doing this?
This article should help:
https://www.gardeningknowhow.com/ornamental/foliage/yucca/divide-yucca-plants.htm