Interesting, I've lived here for 25 years and I never felt that the property was wet, but I do live in Seattle, and horsetails abound, just not in my neighborhood up til now.

Perhaps there has been a change in drainage due to nearby street or home construction.

Ugh. I’m in Seattle too and they’re the worst. I’ve done a ton of research on this and the consensus seems to be roundup and other topicals don’t work that well because they have deep, deep roots. It’s a long slow process to get rid of them, but the winning recipe seems to be:

1). Cut/weed whack them off to deprive of photo synthesis and decrease spore production. You have to be religious on the new spores in the spring.

2). Improve soil, using lime and fertilizer—horse tails like bad soil

3) improve drainage—though like you I have them growing where there are no apparent issues on this.

I started this line of attack last year and there do seem to be maybe 10% less this year.

Yikes, 10%, this is going to be a battle.

And that’s being generous and not counting the new bed they’ve spread to where I didn’t have them last year!