Calling all armchair generals! Boudica's Last Stand.

[Renatus]

Although we also have to bear in mind the rest of the statement. They were 'careless about sowing corn, people of every age having gone to the war, (or 'turned to war'?) while they reckoned on our supplies as their own.' (omni aetate ad bellum versa, dum nostros commeatus sibi destinant.)

So they were not neglecting their crops because they were about to embark on revolt, but because it had already started and they were not physically present!

This is, I think, Tacitus pointing out that barbarians were essentially childish folk who go rushing off to war while neglecting the important things of life. But he is also, incidentally, telling us that the war was in progress during what would usually be the sowing season. So the fighting was happening during either March or October; if the revolt started during the Roman campaign season, which of those options seems most likely?

This is reading too much into it.  Tacitus is simply saying that the rebels went to war without having planted crops.  The point is that they were 'negligent' in having done so.  It could be argued that, if they they had gone to war before the autumn planting, they would actually have been being sensible.  If they had delayed until then, they would have had to conduct their rebellion during the winter months, when Suetonius was no longer distracted by his Welsh campaign and was back in England with all his legions and ready to snuff out the revolt before it really got going.  The March and October alternatives are a false dichotomy.  The fighting did not take place during the sowing season but during tha growing season.

As for crop patterns, the bit I quoted from Shimon Applebaum in this post seems quite persuasive. Britain was exporting grain to Gaul in this period, so the Britons would hardly still be using a prehistoric summer cultivation pattern; winter grains were far more likely the main crop, as they were elsewhere in Europe at the time.

I am not sure that the Scots, who apparently harvest in the autumn, would appreciate being referred to as 'prehistoric'!  However, seriously, if the Britons planted their main crop in the autumn with a subsidiary planting in the spring, they could well have had a surplus to export to Gaul.  They might have had two harvests, a main one in summer and a lesser one in autumn, or, if the crops planted in the spring were properly managed, they could have been ready for harvesting in summer as well.  On the other hand, if crops planted in spring were neglected, as they would have been had they been planted immediately before the revolt, they might not have been ready for harvesting before autumn but would then have been available to sustain the defeated rebels over winter.  The point is academic, of course, because there was no such planting.