Q.Horsetail (Equisetum Arvense). How Do I Eradicate This? Bad Infestation
Am about to sow wild flowers in a previous vegetable patch and have dug it up already. However, Could not remove all of the Horsetail rhyzomes (?) because they are too deep and I know that the remaining small pieces will still grow. Can you recommend anything that would kill these and yet allow me to sow the seeds in early April?

Certified GKH Gardening Expert
This is a tough one. Eradicating this weed is a monumental task, and doing so while leaving the soil suitable in that timeline will be quite difficult. The first thing that comes to mind, that would suit your needs, will be boiling water!
Just dig out what you can first. Then, pour as much boiling water as it takes to cover the area. This will kill everything growing in the soil, usually.
If this fails to work, you will need to turn to chemical means. This may leave you with a spot that is unusable for a little while, though.
Here is an article that will help: https://www.gardeningknowhow.com/edible/herbs/horsetail/horsetail-weed-control.htm
If all else fails, you may have to take an "if you can't beat 'em, eat 'em" attitude. Though the plant can be toxic, young shoots can be eaten. The plant can be useful in other ways, as well.
These articles will help, if it comes down to having to live with the plant: https://www.gardeningknowhow.com/edible/herbs/horsetail/horsetail-herb-growing.htm
https://www.gardeningknowhow.com/edible/herbs/horsetail/harvesting-horsetail-herbs.htm