That plant will have pretty deep roots on it does root. It will keep itself in at that point. At least the myriophyllum species I have does (they all look pretty similar to me). To encourage this, snip off the bottom up to the node, pull all the leaves off the first node of two, and then plant.

This is a great article with pictures I found helpful:
http://www.freshwateraquariumplants.com/aquarium_plants.html


To keep it from floating in the meantime, you can use lead plant weights from the fish store. I don't recommend that, since I personally don't want my hands in water with lead leaching into it. You could try these:

http://www.aquariumplants.com/Professional_Plant_Anchors_on_sale_reg_2_29_p/pr1325.htm

or this DIY project

http://www.plantedtank.net/forums/substrate/47738-how-plant-plants-sand.html

Be aware, I have not personally tried either.

You might also want to look at your substrate. I'm wondering if you have too little substrate and/or a substrate that is too coarse. I think your substrate should grip better, unless the fish are nibbling at the plant or you have fish that uproot plants with their big goofy bodies like my clown loaches!

 

i have no problems with any of the other stem plants i have - baby tears, cabomba, limnophila, so i dont think my gravel is too coarse, probably between 1-3 mm

i am also not too keen on keeping anything lead in my tank...

i also have clown loaches, but they are still quite small, and dont seem to bother any of my other plants.

foxtail just seems more buoyant than most plants i am used to, i dont want to put plastic plant holders, velcro or pots in my tank either, any other ideas?