I'd probably divide them next Spring. Keep one; give away or sell the rest.

How 'cold' do you keep your greenhouse? No, they're not as hardy as Trachys but they should be okay in a chilly/cold greenhouse that is essentially frost-free. I overwinter my big boys in a chilly garage (35-60 in the Winter).--No problems! Keep dry.

I actually prefer to keep mine in a cool place since mine has a tendency of making new flushes indoors (despite never being watered and being away from a window) and when it flushes indoors, the fronds are very long and cannot tolerate sun. Sagos can even handle freezing temps. Unless your greenhouse goes below 30F, it should be fine.
Good luck!
-Alex

Thanks for the replies.

UNHEATED greenhouse, except for some CFC bulbs a la the 'Artictropical' method that Kevin from N. Utah has shared with us on so many threads. Trachies did great last winter this way, but I just don't think a sago would make it.

Alas, parting is such sweet sorrow, but next year, they'll probably just have to go. I can always pick up some new ones the following spring at HD or Lowe's.

What's your gardening zone? What are temps in your greenhouse? Average max/min? Extreme minimum? I can only speak from personal experience. I'm in zone 7 and have mine (must have four, larger tripple-planted specimens) in a large, detached, loft garage which faces due south. The worst of Winter is usually January. I run a fan and a small space heater--just to keep temps above the level of frost. Minimums in January are around 32F inside on very cold nights but days may not get out of the 40's at times of cold and clouds. Milder periods (even in January), can push temps through the 50's into the 60's (maximums). Never have any cold damage despite the fact that the cycads are on the ground and not raised. At the same temps I also overwinter large queens, Phoenix, Chamaerops, more tender species of Trachycarpus, Sabals, Chilean Wine, and Butias. Among the cycads, I have also overwintered Dioon and Encephalartos. I do however, have a problem overiwintering the Cardboard Zamia in these same conditions. This year, I may push things by seeing how long I can keep my Spindle and Triangle palm out there--they too have gotten big and I would prefer not to have to bring them in. Right now though, all the more cool tolerant palms and cycads are still outside as temps minimums have been no lower than the 40's--hardier tropicals may remain until December when temps really start to drop to frosty levels. PS., It is important to keep the cycads DRY if you decide to leave them in your cold greenhouse.--Mine are too massive to even consider bringing inside.--Besides, inside they get mealy bugs and scale big time! Good luck!

You must have had them for quite a few years for them to get 6 feet tall. Do they have pups? If so, try rooting some of the pups next year - it's much cheaper than buying new plants. I have a sago that I've kept in a pot for over 13 years and it's only a couple of feet tall - including the fronds. And it is sporting several pups. Think "bonsai" when you start over. Sagos grow so slowly they don't need a huge pot. Mine is in a pot a little taller than, and about as large diameter as, a one-gallon pot.

I have 4 Sago's I have had now for almost 15 years now in pots and they are pretty ginormous now and keep them in my unheated garage which can dip down to the mid twenties and have never had any problems, they can easily take temps down to 18-20 degrees, the frost I believe is what will damage them more so than the cold.Keep them on the dry side and you should be ok.

Yes, I keep my large sago outdoors until nights get into mid to upper twenties, then move it into the cool garage until the next mild spell, then back out again. They really don't need that much light. The cool temps keep them from flushing as well. I have to use a hand truck to move it. Weighs a ton! I used to be able to hoist it up into the box of my pickup in the garage. Took less room. But not any more!