Some links you might want to check:

http://www.weedalert.com/weed_pages/wa_horsetail.htm
http://www.spokane-county.wsu.edu/spokane/eastside/Weed_Information/weed_text/Horsetail.pdf
http://www.ubcbotanicalgarden.org/forums/showthread.php?t=6795
http://www.hillgardens.com/horsetail.htm
http://www.agf.gov.bc.ca/cropprot/hrsetail.htmhttp://oregonstate.edu/dept/nursery-weeds/weedspeciespage/horsetail/Equisetum_arvense_horsetail.html

Horsetail stands out among other plants due to its perfumes
rare origin and little similarity to most of the inhabitants of flora kingdom. First, handbags
it is the descendant of the plants thriving in Paleozoic era (more than 400 million years ago), and second, it belongs to the Equisetum genus, the only existing in Equisetaceae family known for vascular plants propagated by sunglasses
spores. Consequently, horsetails are closely related to ferns, and similarly to the latter are considered to be non-flowering. The common name of the plant is based on its furniture
physical resemblance to the tail for the horse, while the Latin term for Equisetum means "horse bristle"

I did the same thing as you husband a few years back. I planted some around my small fish pond and it absolutely took over. I eventually dug up the whole bed (back-breaking), and now I'm down to pulling up the rare survivor. It escaped into the grass, but seems to be killed by broadleaf herbicide (if I recall correctly). You might try different herbicides on small patches. I also put in some pennywort at the same time -- what an idiot!

I am on a steeeeeep hill and fighting erosion. I still have to see anything at all invade my yard. Slope, deep shade, red clay (naturally) and close proximity of walnut trees. Nothing will grow here. Maybe this horsetail is the solution? I remember seeing it in its natural habitat, lots of times, and it was very modest.

use it in your compost

Yes indeed - Horsetail Ferns contains Silica at 60,000 - 97,000 ppm natural immune booster

Here's a few other benefits from this plant material:

Silicic Acid - 80,000 ppm (rooting agent)
Phosphorus - 14,762 ppm
Calcium - 24,000
Magnesium - 4,730 PPM
per LD

use it in your compost

Yes indeed - Horsetail Ferns contains Silica at 60,000 - 97,000 ppm natural immune booster

Here's a few other benefits from this plant material:

Silicic Acid - 80,000 ppm (rooting agent)
Phosphorus - 14,762 ppm
Calcium - 24,000
Magnesium - 4,730 PPM
per LD

We are new to the plant, but there were already two large pots of this beautiful plant on opposite sides of a small patio. Each looked lush, was thick and about 3' - 3 1/2' high. After the spring rains and a few weeks into the hot summer, we watered the plants substantially, thinking they did great even when the roots were under water.

Sadly, we ended up with brown hard stems -- those that weren't still vertical were bent over.

Is there anything we can do to salvage the plants?