It's paw paw weather! The heat is upon us in the Pacific Northwest on this late summer day. Paw paw fruits need heat like this to ripen.
The oblong, green paw paw fruits are growing at Raintree. I was just checking on them... As orchard manager, much of my work at Raintree occurs in our orchards. I also have the opportunity to work in our greenhouses, learning from our horticulturist.
One of the tasks I've helped with in the greenhouses is grafting paw paws. I am a fairly novice grafter, having only begun to learn since starting at Raintree over a year-and-a-half ago. This post will describe my learning process and share some of what I know, though of course I'm no expert yet! I'll focus on my paw paw grafting experience here, but I would like to continue to learn more about grafting in general.
The idea behindgrafting is essentially to join two plant stocks together. Scionwood from a selected variety (chosen for all kinds of reasons, but common lyits fruit) is put onto a selected rootstock. A rootstock might be selected for its vigor, disease resistance, adaptation to a certain environment, or other reasons.
Vegetative buds on the scionwood need to be distinguished from flower (aka fruit) buds in paw paws because their buds grow deterministically in that way. A vegetative bud will grow into a leaf or a
stem, whereas a flower bud can only become a flower and then a fruit. Paw paw (the main species being Asimina triloba) are tropical-seeming plants in part because they bear fruit directly on the stem. To humor the botanist in me or the reader, I'll throw out the term cauliflory. Indeed, paw paws are tropical. They're the northernmost member of the custard apple or soursop family (Annonaceae) and the only one that has escaped the tropics. Paw paws' champion fruit ranks as the largest edible fruit in temperate North America. The broad-leaved, shrubby, clonally spreading paw paw ranges in the East all the way up to Ontario, Canada. Although the fruit's perishable nature has precluded its appearance in grocery bins, local economies exist around paw paw patches. Paw paws have made something of a cultural impression, if only regionally:
Paw paw flowers appear before the leaves in the spring and are borne directly on the stems.
Anyway,I could go on about paw paw natural history for a while, but let's circle back to that some other time. For now, onward with the grafting!
We use whip-and-tongue grafting to fuse together pawpaw rootstock and scionwood grown in our orchards. I match girths (orcalipers) of rootstock and scion as the first key step in the process.The rootstock and scion should be as close to the same thickness as possible. Then, I cut upward at a steep angle on the rootstock. I make sure to have the flat side of the grafting knife towards the base of the rootstock and the beveled side away, ensuring a clean surface for fusing to the scionwood. Then, I do the reverse to the scion stick, cutting downward at a steep angle away from the top. Again, the flat side of the knife makes a clean cut on the side of the scion to join with the rootstock. Cutting inward into both scion and rootstock allows cross-hatching them for strength. The pieces slide into each other. The main point is to have the cambium layer (actively growing tissue that generates xylem and phloem) of both line up as much as possible. For this to work, the cambia have to fuse together. Tape, band, parafilm, or the like gets wrapped around the join (the graft union) to help holdt he two pieces together and prevent drying out. A dab of wood glue to the top of the freshly cut scionwood (cut back so that only one or two vegetative buds are left above the graft union) helps ward off desiccation as well.
Fig 1. Paw Paw seedling with bypass pruners, wood glue, and a grafting knife.
Fig 2. Matching cambium of seedling and scionwood before grafting.
Fig 3. Finished whip-and-tongue graft.
Grafting is both art and science, I think. I'm eager to keep practicing to improve my art. I have worked some with whip-and-tongue on other species. I'd like to learn other styles of grafting, too. It's amazing that two (or more!) individual plants can be joined together with such simple technology, as people have done since Roman times!
Happy growing! Because, after all, change is the only constant. -Xander Rose