One of our favourite group of plants is the Pouteria genus. Ever since we first tasted fruit of a delicious lucuma, we were captivated. In that time we’ve gathered many Pouteria species to work on breeding and selection work, including lucuma seedlings from 7~ different sources. Lucuma we have found grows excellently for us so far, with most seedlings having no trouble at all during Winter. In fact we can probably grow lucuma better than Citrus on their own roots! (Citrus is the commonly used benchmark for whether you can grow lucuma or not)
The next most hardy Pouteria species available to New Zealand growers after lucuma, is green sapote (Pouteria viridis). There are fruiting trees of green sapote in Auckland and Northland that we are aware of (we have just sprouted some seeds of one Northland green sapote). We initially treated our green sapote seedlings as we did for lucuma – just leaving them outside over the previous Winter, unprotected other than being in a sheltered location protected from wind and basically frost-free (we will write future articles on the merits to this approach, but the tl;dr is the smaller cell sizes of outdoor grown plants are much more resistant to frosts, and the stress wood grown by plants exposed to wind prevents them from snapping in high winds, unlike more brittle indoor-grown plants). However the sheer amount of cold and wet conditions over the previous Winter took their toll, and these few green sapote seedlings defoliated and had the growing tip die. 2/3 did survive however, and eventually began to regrow – but the regrowth was very slow and relatively late. At this rate it would be 2 steps forward, 1 step back for many years to come. Surely there must be a better way…
Well we decided to experiment with grafting green sapote onto lucuma rootstocks, since lucuma is so hardy for us and seems to start growing in cooler temperatures than the green sapote. There are older reports from Florida of mamey sapote being successfully grafted to canistel, which gave us a great amount of confidence to proceed. We can already graft avocados, so why should Pouteria be any more difficult? Well Pouteria have a trick up their sleeves… when cut, they release a white milky latex, which quickly seals off any wounds and prevents cambium contact – this makes grafting them quite difficult (a friend reported about a 3% success rate using standard cleft grafts…). There are supposedly ways around this, but they are not particularly easy or a high success rate (a problem when you have limited scionwood as well). Instead we turned to a very old and somewhat underutilized technique – which is approach grafting, in order to surmount the latex problem. We just used a standard approach graft, and there are many good articles on how to do this already – so I won’t cover the technique in detail.
This particular green sapote was grown from seed imported from Hawaii, sown in early Spring. It grew very well – thanks to the Summer being so warm and a favourable spot. The approach graft we performed in December 2021. The two plants had to have their stems aligned (thanks to the help of a cinderblock, this was possible). Two long cuts on the stems were made, about 4cm long, and then the wounds matched together and tightly bound with grafting tape and garden twine. The plants remained outside in mostly all-day sun after this, since they both had fully functional root systems.
Some time in February 2022 I removed the bandages and checked for signs of a graft union – thankfully it appeared to be healing. The first sign was that the two plants didn’t spring apart – they had been tied together under tension. I re-applied new bandages and left it again. In late February 2022 we began the slow process of detaching the green sapote from its original roots. This involves making an incision through the original green sapote stem underneath the graft, and deepening this cut every few days (eventually the cut is all the way through), and watching for signs of water stress. It’s very important to note that the smallest sliver of attachment between the graft and the original roots can provide a lot of water – don’t be hasty removing the last part. My final cut gave the plant a fair amount of water stress – the leaves wilted significantly, and I had to bring it inside, chop some leaves off and mist it for a few days until this was resolved. It’s now been over a month with the grafted green sapote being outside, most of the days being quite sunny, and the green sapote is back to growing again (albeit slower than at the peak of Summer as we move into Autumn).
I will probably protect this plant over Winter (bringing it inside when necessary) until next Spring, as the green sapote part of the plant isn’t woody yet. There’s still of course a possibility of delayed graft failure – so it will need to keep being monitored. The reports of mamey grafted onto canistel said delayed graft incompatibility didn’t appear to be a problem after a few years – so we are confident it will be okay.
That’s about it for this research update. If you are interested in collaborating with our Pouteria research project – please send us an email. We welcome inclusion of extra materials and ideas. 🙂
