What wire do I need for a 60-amp sub-panel in a greenhouse ran 140' direct burial from the power pole?
Fat conduit and fat panels are your friends here

Given that you've already gone through all that work to dig a 140' trench, why would you want to have to do that again if you ever need more than 60A at your greenhouse, as you would if you had to replace a direct buried cable? PVC conduit is not much more expensive, allows you to use a slightly shallower trench depth (21-22" instead of 25-26"), and provides you with the ability to have much more expansion room in the future, as a 1.5" PVC conduit can easily accommodate a 125A feeder.

Inside that conduit, 3 6AWG stranded copper THHNs (hot, hot, neutral) with an 8AWG or 6AWG bare copper grounding wire (to return wayward utility electricity to the utility) are the simplest way to ship 60A out to your greenhouse.  You'll of course need the correct 60A two pole breaker for your pole's panel, as well as two  8' grounding rods driven 8' or more apart at the greenhouse and connected to the subpanel at the greenhouse with 6AWG bare copper to form a grounding electrode system so that wayward natural electricity has a path back to nature.

Furthermore, when you're going subpanel shopping, there's absolutely no reason to skimp on the subpanel, as again, saving a few bucks now is not worth the hassle a full panel causes down the road.  You'll want to get a 24-space or 30-space, 100A or 125A, main breaker panel for the greenhouse and fit a matching accessory ground bar or two to it as well as removing the neutral-to-ground bonding screw or strap from the panel.  This way, neutral and ground are separated at the subpanel, as they should be, so that normally flowing power doesn't try to return on the grounding system and cause problems in the process.