As you can see below I've tried to tie down the stem better to reduce stress but I'm not sure how effective it will be.

Sometimes it just happens. Did the mix in the container go through a really dry period and then get a whole bunch of moisture dumped into it suddenly? The xylem can swell with the excess water and split the stem. The good news is that tomato stems are very resilient, vertically oriented damage doesn't cut off the flow of water and nutrients much, and as long as you support the plant well, it shouldn't affect your production. The biggest danger is if a fungus or mold takes advantage of the opening and rots the plant. But things look OK there so far.

Thanks Dave you're the man seriously, I take your wisdom like it's gold.

Thank you too gorbelly, and thanks for the hope. They did go through a dry spell, 4 days with 86-92 degree weather. I lost 6 tomatoes to BER:/ because lack of water, so you have to be right on the cause!

Growlove, why not try wrapping a bit of electrical tape around it.... that's what I would try. Others are probably right though, it will most likely heal itself and won't effect the plant adversely. Good luck.

Harry

I wouldn't wrap it, especially with anything nonpermeable. Sounds like a good way to create an environment for mold, etc. Better to give it airflow and let it heal up that way, IMO.

Agree. You want it exposed to air so it can dry out and scab over.

Dave

As long as it doesn't bend in that place and breaks, it will be fine. Even if it remains like this, the growth will be unaffected.

I've had tomato plants soldier on through an over 50% severed stem like nothing happened. I've seen parts of plants hanging on by what looks like a flap that still produced after the break.