I've found a packet of spinach seeds I 'forgot' to sow earlier in the year. Latest sowing date is May on the packet.
What will happen if I put them in now? A late crop if the frost doesn't get it? Or will they have a short growing cycle and run to seed when a few inches tall? (They seem to do that anyway....)
We regularly plant a crop of spinach late in August and sometimes, just for the heck of it as late as early September... if we get (I'm in the western U.S.) 4 weeks of good cool weather with no frost, we get a nice harvest of crisp spinach... I'd encourage you to go for it. Many of the insects that ravage leafy crops are gone and as long as you keep it well watered, I'm sure it will do well... Good luck!