A healthy rootstock is essential, more important even than the state of the scion as the latter has no roots for pathogen entry and foliar sprays can take care of the leaves. Cuttings for grafting rootstock should be taken from plants that are proven free of diseases as these can easily return to bite you just when the scion starts to establish itself.
The established cutting to be used for grafting should have six to eight leaves – at grafting time the bottom two leaves will be retained while the rest will be stripped off to allow space for the scion and bindings. Naturally a too short cutting, unusable for grafting immediately will need to produce new stems that need to harden off, adding to difficulties with nursery space and time, however these give good results when the new stems have matured enough. These new stems are usually soft and do not slip the bark easily if the rind graft needs to be used, so are better with cleft or splice grafts.
Grafting on to unrooted stock does work sometimes and is operator friendly, but only under optimal conditions otherwise failures will occur frequently, so this procedure for the rarer species taken from the wild is not really an option. A cutting that starts rooting only after six weeks or more unacceptably increases the risks to the scion especially if the scion is from the wild.
I have recently grafted twenty eight Mimetes on to unrooted Leucospermum stems as a last chance attempt. However I first stood the unrooted stems in 2% sugar water for twenty four hours and the scions for four hours, grafted them and placed them under grow lights (16/8 hours photoperiod) of red/blue wavelength, watering them as needed with no misting as they were in an enclosed area. After twenty days there was no wilting but roots will only begin to show after fifty days so at the moment I am still waiting, update to follow.
Grafting on to well rooted cuttings before potting-up is rewarding, even preferential for commercial nurseries, however the roots should be well established. It seems that this type of cross genus grafting requires a well rooted stock to support it over the first few weeks at least, and this should apply for other rare species unless the scions are from a manipulated mother plant. Newly rooted cuttings often do not slip the bark easily for rind grafts, but performs better a few months after establishing a good root system.
A scion that has failed to form a callus with the stock can be pruned off and the stock potted up for new shoots to develop as these are excellent for later grafting, however there may be a co-morbidity in the rootstock so a suitable quarantine time with anti-fungal sprays is advisable before the next grafting attempt.
Established rootstock that is growing strongly may produce two or more shoots so the weakest can be pruned off. If there is plenty of scion material multiple grafts can be done on the same stock. I once grafted three different Mimeteses (M. argenteus, M. hottentoticus, M. splendidus) on to the same Leucospermum rootstock that had three good stems but I think the stock got confused with the different messages as growth flushes of these different scions occur at different times so after a while it gave up. Three different colours of leucospermum on the same rootstock might look good!
I have tried various hybrids of proteas and leucospermums and they all seem suitable as rootstock, so it comes down to what is readily available. I have mature mother plants of Protea Pink Ice and Leucospermum Veldfire. They root and receive the scion well, continuing to grow for years when planted out in an environment full of pathogens (Phytopthora, Colletotrichum, Fusarium, Dreschlera, Ilyonectria, and others). They carry well but are not complete proof against these pathogens, so some do succumb.
There are hybrids developed for specific resistance to nematodes, for compatibility with high or low Ph or for resistance to certain pathogens. I have a high incidence of Phytopthora cinnamomi and lose grafted plants quite regularly which proves that there is no proof against the “Big P”.
Quite often the rootstock, although compatible with the scion, after a few years the scion starts losing vigor and I put this down to the rootstock not delivering the correct nutrients that the scion evolved in. (Protea odorata, mucronifolia). The answer may lie in a different rootstock more closely adapted to the environment of the scion and I need to test this. A more immediate solution is not to plant them out but to keep potting-up the grafted plant as it grows over the years and manipulating the soil by way of changing the ph., additives etc.
I have recently added to further strengthening of the rootstock by flooding the medium a few days before grafting with sugar water at 2%.