I decided to write this article about grafting cacti after writing about my Trichocereus Peruvianus clone, “Ferret”.
So if you haven’t read about Ferret then follow the link as it will put the following more into context.
But first here is a quick refresher.
Both these photos were taken on the same day.
The graft on the left was a tip graft. Over a 12mth period it grew 35cm. It also had the bottom cut out of the plastic pot so the roots could grow down into the soil below.
The graft on the right was a small slab graft. The scion was taken from just below the tip which was used as the scion on the left. The rootstock was also contained within the pot, unlike the tip graft. In fact the rootball wasn’t even that established. As you can see I had to stake the one on the right. It wasn’t falling over but it was getting quite top heavy, so it probably wasn’t far from toppling over. It had grown 55cm in the same amount of time.
So this is the topic of this article!
Why did the scion on the right outgrow the one on the left?????
Initially my expectation was the tip would outgrow the slab. That’s why I cut the bottom out of the pot and then placed it over a very fertile soil. The plan was to give the tip the best opportunity to put on as much growth as possible. Whereas the slab graft was placed against a fence, surrounded by other pots and in effect was a little bit neglected.
In my previous article about Ferret, I proposed that because the tip graft had roots growing into the ground that the rootstock devoted more energy into growing the roots, rather than the scion. And in some respects there is probably some truth in this as well. But in this case, the slab graft had outgrown the tip by about 20cm.
So there must have been a more definitive reason!
But first here is some grafting “theory” that will hopefully lead to a feasible conclusion. This is important as it will hopefully complete the picture!!!!!
I’ve watched quite a few grafting tutorials on Youtube about grafting cacti. One of the first I watched stated the importance of getting the outer edge of the two vascular rings to overlap. So the theory is the ring on the scion needs to overlap the ring on the rootstock. Just like the rings on the Olympic emblem, if that makes sense!
So with all my original grafts I tried to ensure that they overlapped. In effect this is easier said than done. Once the scion is placed on the rootstock you can no longer see the rings. So in effect, you are just guessing and using judgement. However most of my original grafts were successful, so I just took it for granted that the rings did overlap.
Then one day I watched a video which stated that it wasn’t really a factor. That they didn’t need to overlap. This to me made sense, as I’m sure with some of my grafts the rings possibly weren’t overlapping. But because they worked, I just assumed that they did.
Take a look at the photo on the right. This was a tip graft that separated while the cut surfaces were still callousing. The tip scion was cut off and regrafted onto another rootstock. The remains of the scion were left of the rootstock and surprisingly still managed to produce a pup.
In this closeup you can see the scion has some contact with the vascular ring of the roostock. But it looks like most of the vascular ring of the scion is now gone. It’s difficult to see but there are now only 3 of the original 6 ribs of the scion remaining. The other 3 were shrivelled by the sun. So now the scion is probably just behaving like an areole graft.
With an areole graft there is no vascular ring remaining on the scion. However, the areoles on a cactus are connected to the vascular ring. So when the areole is grafted onto the rootstock it would be very difficult to line up the connection with the outer edge of the vascular ring on the rootstock. Therefore, it just contacts somewhere within the vascular ring of the rootstock.
Therefore it appears what is important is, the scion just needs to be contact the vascular ring on the rootstock.
So lets now take it one step further . . . .
Let’s propose that whether or not they overlap determines something else. Not whether the graft will take and the scion grow, but more so, how fast the scion grows.
Or, put simply! If you can get the vascular rings to overlap it will increase the ability of the rootstock to feed the scion!!! Hence a quicker growing scion.
So to get back to the original question!
Why did the slab graft far out grow the tip graft?
In this instance the tip scion was very small. So the vascular ring would also have been small as well. I’m guessing there was little overlap of the outer rings, or none at all. In fact, it was more likely to have been a small circle place inside a larger circle.
However in the case of the slab graft, I’m guessing there was more overlap of the outer edges of the two vascular rings. By it’s very nature, the slab graft looks like 2 parallel lines overlapping a circle. If done right you can get 4 points of contact. With a tip graft, the best you will ever do is get 2 points of contact (between 2 circles).
The other point about a slab graft is you can manipulate the width of the vascular ring. If the vascular ring on the rootstock is small, then you can slice the scion to make those 2 parallel lines closer together and then aim for the 4 points of contact. If it’s large on the rootstock then you can afford to leave the slab scion larger (which is probably better!).
Conclusion
It’s always hard to prove why one graft will outgrown another. In this case both grafts were done at the same time and the scions were taken from the same plant. The rootstocks were both Spachianus and very similar. The main difference being, one was a slab and the other was a tip scion. From practice, I’ve found it much easier to align the vascular rings on a slab graft.
So maybe it’s not a case of a slab graft being better than a tip graft. But rather a case of the slab graft just makes it easier to overlap the vascular rings, plus, potentially getting 4 points of contact.
Therefore, to address what really started this all off. One grafting video stated it was essential to get the vascular rings to overlap while the other said it didn’t really matter. Both contradictory, but essentially they were possibly both correct . . . . to some degree.
So my conclusion is this. To get the graft to take, it doesn’t appear to matter. But to get the scion to grow optimally, overlapping the the rings does appear to help!
Try it out . . . . and let me know!
Read more about Grafting Cacti. Grafting Cactus Seedlings
