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| Bonus: leftover tatami bits feed the soil. |
So, one year ago I got brave enough to ask our local tatami master if I could have some old mats. He said yes and a friend helped me haul them to the garden and lay them out between my rows. I still use plastic sheet mulch for the plants themselves as organic matter, like leaves and straw, can be hard to come by at the right time. A solution for that also exists, but I've not discovered the right one for my garden just yet.
The mats blocked weeds effectively while letting water soak through, and they staved off some of the erosion that commonly occurs with Tokyo's high winds and heavy rains. They did blow about a bit in the occasional typhoon, but overall they survived quite nicely. When I made my latest lasagna bed - committing a whole long row to what I firmly believe is the best idea ever for building soil vitality - I used the mats to cover the whole of it twice over. (Some mats stayed between the rows, but others had to be taken up when Takashi-san plowed and set the plastic mulch in place.) It helped keep the beds warm so active decomposition could occur, and it again kept things from eroding.
The benefit of the mats that I didn't anticipate was their decomposition. As the mats break down the igusa (grass used specifically for making tatami) slips away from the string that previously held it all together. As I pick them up to move them from place to place, the igusa is often left behind as a valuable soil additive. The nylon string is proving something of an annoyance, but I'll just ball it up and bag it with the mint on burnables day. The old tatami themselves, I've decided, will go at the bottom of the compost bin when I turn it in a few weeks time. Or I'll experiment and slip them between layers of turned compost, perhaps, as a different sort of lasagna bed. Oh, the possibilities!
Got an experiment in repurposing that worked out well? Even if it didn't that's ok. We've all been there. Let's hear it either way!

Comments
Finally, before you use the tatami you might want to make sure it is not treated with any chemicals. Some of the cheaper tatami mats might be.
As to your question if it would be too heavy for your seeds, my hunch is that yes, it would be. I use old plastic grocery bags to make a quick and dirty greenhouse for sprouting, although a glass bowl would also work. You just need to keep them cozy until they sprout.
Kate, I totally understand! But you know, you might want to look into what native species grow in your area and work from there. Why make more work for yourself if you could have a beautiful prairie or other cool planting? Seriously, it will be less work, benefit pollinators, save water and energy, and look awesome!
Kevin, Thanks for the good advice. I need to learn how to ask that question in Japanese.