Not much you can do against nematodes other than to use clean plants in containers that never touch the soil.

It would be a good idea to confirm what your problem is. Can you post pictures, please?

Nematodes can be beneficial, most likely, or harmful, some. One of the most effective ways to control the bad nematodes is to add organic matter to the soil which will increase the number of predators of the bad nematodes. Some people buy nematodes to help control other soil dwelling pests.

Here is a link that might be useful: Nematodes

lily, the only way to find out if you have problem nematodes is to send plant AND soil samples off for testing. Call your extension office for information on how to take the samples and how to package them. They have to be handled carefully so that the nemas arrive at the testing site alive and well.

Are you thinking of foliar nematodes or those that infect the root system and are found in the soil? I 'assume' that you've been reading information on foliar nematodes, since you mentioned hosta nematodes (which are foliar). Ferns, too, can be carriers of foliar nematodes. Many plants, too.

However, some of the other plants you've mentioned can be prime soil nematode targets. The boxwood, in particular.

As Jean says, there are no chemicals you can use, organic or not. Nematicides are available only at the commercial level, and for specific uses/locations. They will never be available for homeowners.

I strongly recommend that you contact your local extension office promptly, telling them that you are concerned about soil borne or foliar nematodes. They will be able to help you with proper collection and handling of the samples. Don't just let them tell you to take a soil test but not a tissue sample....the foliar nemas will be found in the plant tissue, not the soil.

Your extension office may also be interested in finding other causes for all of your plant problems. Severe and wide-spread nematode damage (at the homeowner level) isn't all that common, even in those locations where these animals are pretty abundant.

We see them a great deal in plant nurseries (growers), monocultural (crops), sod farms, and similar. Serious problems in our backyards are almost always due to the introduction of infected ornamentals, shrubs, and/or sod.

IF the test results indicate a serious population of these parasitic nematodes, then you'll probably need to evaluate your gardening strategies. But we don't need to go into that now. It shouldn't take too long for your nematode assays to come back from the lab.

I garden in 4 x 12 box sections. Last year what I believe to be root knot nematodes attacked my okra and wiped them out so I grew my okra in a box on the other side of the yard this year and they showed signs of being infected. I pulled them up yesterday and sure enough they were. I put the root balls in a black plastic bag and have it out in the sun...hopefully getting some small measure of revenge. I must have spread them on my gardening utensils although I thought I was being careful. My neighbor told me 30 years ago we had nematodes and was why he stopped gardening. I haven't had a problem with them that I know of over the years, but then I have only raised okra for a few years. Okra seems to be the Pied Piper to these things. I will not plant again.
So now I have less than a month before I will begin fall planting and still have this issue. I have the box that was infected last year under clear plastic since spring, the box adjacent to it is planted in tomatoes and will not be available for any sort of treatment this year.
The section where the okra was this year is about 4 x 8. Because I garden in boxes I can shovel the soil out to a depth of perhaps 10 inches. Gardeners here have noted that by treating soil at a very elevated temperature both the bad and the good things in the soil would be destroyed. Ok. I assume it would replenish itself naturally plus the addition of my compost pile should be enough to kick it along for the fall. But what temperature is the right temperature? I believe I read somewhere that 150 degrees over 6 weeks or so would run them down deep. I don't have that long.
My plan at the moment is to spread black plastic next to the box, as much as needed to limit the dirt from the infected box to about an inch, shovel it out on the plastic and probably cover it with clear plastic to create an accelerated heating condition. Solarizing in ground only moves the nematodes down about a foot from what I have read. I don't want to give them an escape path.
Does this sound reasonable? Has anyone tried this?

Nonikiyu