Safely culling water plants
Good day. I have a small water garden and I need to safely dispose of excess salvinia, frogbit and water hyacinth. As these plants are potentially invasive, I want to know is it's safe to compost them. I am aware that they won't survive in cold temperatures but was not certain if it got cold enough in Portland to kill them off in a compost site. I have considered setting them out to dry prior to composting but wonder if the seeds would still be viable. Thank you for your time.
It's great that you're taking the trouble to dispose of potentially invasive plants correctly. The best and safest way to dispose of any invasive plant material is to bag it and put it into the garbage - not compost it. The seeds would probably survive composting. It would be very unfortunate to get any of these plants into our streams and wetlands. All of them have the potential to be highly invasive.
Salvinia (Salvinia minima) is listed as "Quarantine" by USDA, Noxious and Invasive Plants in Oregon https://plants.usda.gov/java/noxious?rptType=State&statefips=41. Frogbit (Limnobium spongia) or (Hydrocharis morsus-ranae) is so far unknown in Oregon, but is highly invasive and considered capable of surviving here, Common Frogbit https://www.oregon.gov/ODA/shared/Documents/Publications/Weeds/CommonFrogbitProfile.pdf. Water Hyacinth (Eichhoria crassipes) is also has "Quarantine" status in Oregon, but there are populations already here, Eichhoria crassipes https://nas.er.usgs.gov/queries/FactSheet.aspx?speciesID=1130, so it is capable of surviving here. As you have noted all are potentially invasive. By taking action you are doing your part to prevent their spread here in Oregon.