From the White Flower Farm website:
"Try it with Monarda or Heliopsis. Lysimachia makes a good cut flower and with Amsonia and Platycodon boasts some of the most striking fall color to be had in a perennial garden."

I've wanted to plant it around my pond. I have Siberian iris, violets, lily of the valley, forget me nots, and blue rug juniper around the pond. I thought the white flowers of the gooseneck loosestrife would look nice with all the other blues I have.

Gooseneck loosestrife does not have buddies, only victims. Its long worm-like roots spread far and wide, both near the surface--sometimes right over the surface--and quite deep. It is almost impossible to get rid of--a years-long project.

I have succumbed to its delightful flowers twice in my gardening life--and rued the day both times.

If you must have it, try it with low shrubs.

Ditto ginny12's post--it's invasive. Once you have it, you'll never get rid of it. White Flower Farm, Fine Gardening and Missouri Botanical list it as aggressive/invasive. It's native to China & Japan and if you haven't yet been invaded by Japanese knotweed, it will find you at some point. My next-door neighbor has gooseneck loosestrife growing by his patio but it has already popped up in several other parts of his garden where it apparently self-sowed via wind or birds. A garden friend in Illinois has been trying to smother/kill it without success for the past several years.

Just to clarify - gooseneck loosestrife is not the same as Japanese knotweed.

I have the same reaction to gooseneck loosestrife as Ginny, however. I have grown it and would never do so again. I don't think it is invasive in the sense of seeding onto wild areas, but it certainly is an aggressive thug in a garden, both overwhelming other plants and spreading by roots in such an aggressive way as to be far to much work to remove.

round up?

Will the gooseneck bother established trees (Japanese maple, firs) or knockout roses??

I agree with Ginny, Gardenweed, and NHBabs. I have gooseneck loosestrife as well (given to me many years ago). Even in the poorest conditions it spreads and thrives. I yank up a lot of plants every year and dispose of them, but I think it will always be around. I don't mind a few, but I have to keep after them. I have japanese maple, knockouts, etc. and I wouldn't plant them in that area. They might not harm the plants, but you will be weeding them out regularly.

Gooseneck loosestrife (Lysimachia clethroides) is an aggressive spreader to be sure but it is NOT a listed invasive.........BIG difference.

Personally, I love the plant and make the time to plant it so that it won't spread beyond its bounds (root barriers, container sunk into the ground, etc.) I have also found that planting it in hot, dry conditions limits its spread significantly. It has never popped up from seed anywhere I have seen nor does one find untended, unplanted clumps of it, so that likely why it is not a listed invasive species. I agree the fall color is striking and I would combine it with any other perennial that blooms about the same time and is also a vigorous grower and profuse bloomer. I like White Flower Farm's suggestion to combine with Monarda - that is also a very aggressive spreader, as is Helianthus 'Lemon Queen'. Plant 'em together and let 'em duke it out! LOL!

I guess it depends on how much one likes the plant as to how willing one is to deal with its foibles............some aggressive spreaders do seem not to be worth the effort. But it's hard not to appreciate that sea of white curving blooms on a well grown clump of gooseneck loosestrife or what it adds to a cut bouquet and I choose to make the extra effort to keep it check for that reason.

I've made the mistake as well on planting it in the garden. One section was totally out of control and I had to dump some weed killer in that spot to get rid of it all. The other spot is somewhat shaded and very poor dry rocky soil. It doesn't spread much at all in this location and give some height and bright white color in an otherwise empty space.

Camassia quamash is a rare native bulb that likes water. Colorful but not super aggressive. There are water-lving varieties of liatris. Not super colorful, but cinnamon ferns and apios americana are aggressive natives that like water.

My first thought about a companion? A Shovel.

A companion? Strategically applied herbicide! Otherwise, you'll be digging it up for the rest of your gardening life! Love the bloom and once had it in a shady site with clay soil where it wasn't such a bully. But in sunnier, well drained soil - I pull it, poison it, and it still shows up every spring - where you would want it least - hale and hearty!