I don't know for sure as I have never had to deal with nematodes, but I would think teh drainage holes would allow them to enter through and the roots of plants would also find their way into teh infested soil below.

I am thinking it probably is a waste of time, but I don't know for sure. I wish you louk, I understand those nematodes can be a real pain to cope with.

holy cow did I need a spellcheckur with that last post ;-)

How deep is your raised bed? I'm not sure if it would work, but if your deep bed is high enough, how about drilling holes in the sides of the deep bed instead of making holes at the bottom.

Thanks to you all for the suggestions. I went ahead and lined the bed with plastic. I took Monika's advice and put the biggest drainage holes in the side (the bed is 12" deep) but when I tested it with a hose I got a little worried because it didn't drain very well, and wound up putting some in the bottom too. I figure it will at least slow down the nematodes, and if I do get them eventually I can always try growing a cover crop of marigolds, or solarize the whole thing.

Thanks again. This forum has been a great help to me getting started.

The biological activity of the compost will effectively combat whatever nematodes are in the soil below. As the chemical and drainage properties of the underlying soil are altered by the leaching in of nutrients and critters from the compost, the "bad guys" will die off and be replaced by a more beneficial population.

Though a layer of plastic would slow down exposure of the mix to the native critter, there really isn't any way to keep the nematodes from drifting in anyway. It is better to go ahead and allow the roots to go for the nutrients and additional water below.

If planting beans or legumes, it would probably be a good idea to innoculate the seeds the first time you plant that crop in a bed, and spread the leftover inoculant over all the beds to spread those benefical critters out.

I doubt using compost will end the nematode problem Ray. Surely the more life that is in the soil, the more everything will eat everything else so the bad nematodes will take casualties, but I have never heard of biological activity from compost or any other organic product making a long lasting difference in nematode populations particularly in areas where the infestation goes on for miles and miles. From what I understand Florida suffers from serious nematode issues so learning to live with it is the key.

It is a matter of degrees. I've also got nematode issues here, but in a raised bed setting, most of the roots are in a soil that has been modified to the point the local nematodes are not suited to thrive in it, and they drop off to levels where they are not an immediate threat. If I took the compost layer away, yes, the nematode issues would return almost immediately. A neighbor down the road (less than a mile) doesn't have to be anywhere as careful as I do when picking fruit tree rootstock because he has clay soil where I have sand, and the root knot nematodes can't build up enough of a population to cause trouble with his kind of soil.

In either event, a layer of plastic isn't going to keep them out, as they can blow or track in from anything coming from the surrounding soil... To use your words, I'm trying to say that you are going to have to learn to live with them in your beds, so you might as well leave the liner off hte bottom and let the plants themselves figure out where their roots do best while moisture can still percolate up from below (and the beneficial fungi can still send their structures down), and instead focus on the practices that will minimize them in your growing areas.