It sounds like something out of a spooky sci-fi movie, but now you can buy and try growing a plant that produces tomatoes on top, and potatoes underground, the perfect plant for the gardener who has everything.
Aptly named "DoubleUP", the Potato Tom is a new release from innovative Kati Kati nursery Incredible Edibles owners, Andrew and Fiona Boylan. They're also the people who brought us the flavoursome, single-bite sized feijoa Bambina, bred to be eaten skin and all.
While the DoubleUP Potato Tom is a unique grafted plant that has, in fact, been done before - although not in commercial quantities - Bambina was selected from seedling progeny of feijoa grown by the Boylans, who now own the plant variety right and trademark name. Already proven to be a home garden hit, the largely self-fertile Bambina ticks all the boxes for pint-sized sections, and the person who likes to grow something a bit different. Which is also part of the appeal of the new DoubleUP plant, says Fiona Boylan.
But rather than some freak of nature, or a genetically engineered marvel, it's simply a seedling tomato plant grafted on top of a potato plant, created using a technique similar to that used for years to produce "supertom" tomatoes.
And the double species works because potato (Solonaceae tuberosum) and tomato (Solanum lycopersicum) belong to the same plant family, Solonaceae, and are closely related, sharing a common basic chromosome count of 12. Although the tomato was previously listed in a separate genus of Lycopersicum (L. esculentum), in recent years it's been re-classified as a Solanum, the same genus as potatoes and indicative of their close relationship, botanically speaking.
But rather than just the chromosome count, it's the compatibility of the graft union, where the all-important cambium (growth) cells found under the skin of the potato and tomato shoots need to match up for the graft to work.
Tomato seedlings were used for the top, or scion, part of the plant, then grafted on to the emerging shoot from a potato tuber to produce the dual purpose plant.
The Boylans chose to use the popular potato variety Agria for the root part of the plant, and paired it with the prolific cherry tomato Gardeners Delight, to produce a predictably cropping, easy-to-grow plant, perfect for pots and patios.
But, as supplies of this new wonder plant are limited, it got me thinking I could try grafting my own at home, using the same technique, or try it with some other Solonaceous plants. Given they include some of our most productive and important vegetable crops, such as eggplant and peppers (both sweet and chilli) I figure I could double my production using half the space.
Checking with the Royal Horticultural Society's website, I find indeed, grafted vegetables of several kinds are nothing new and, in the UK where eggplants and both sweet and chilli peppers are often grafted, growers use rootstocks of specially bred F1 hybrid tomato varieties Aegis and Estamino. Similarly, cucumbers, melons and watermelons in the UK are commonly grafted on the cucumber variety Triumph.
Grafting brings the benefit of disease resistance, where the rootstock is a hardy, vigorous and high health variety with tolerance of, say heavy soils, but which may not produce such a prolific or palatable fruit like that of the desired scion (top growth variety).
Grafted trees are also nothing new and are used to grow almost all production fruit trees, as well as many ornamentals such as flowering cherries, elms, maples, honey locusts (Gleditsia) and different varieties of beech. Roses, too, are grown with a graft that uses a single bud, hence the term budded - as opposed to grafted - roses.
And grafted trees can induce precocious (production when still young) flowering, as with say Magnolias, or crops, as with fruit trees.. Grafting can also be used to overcome a juvenile stage of a plant, a trait common among New Zealand indigenous plants, for example lancewoods, kowhai and kauri.
While lots of home gardeners will have had a go at grafting fruit trees, it's probably in the vege garden where you could most easily have a go at grafting - not only tomatoes, but also, perhaps, eggplants on potatoes.
Checking the genetic lines of Solonaceous plants, though, it does seem that as eggplants (Solanum melongena), are more closely related to potatoes than sweet peppers or chillies (Capsicum annuum), they are probably the most likely grafts to work. A graft of peppers on potatoes would require a match between different genera, whereas that with tomatoes, eggplants and potatoes are between the same genus.
But intergeneric grafting does happen. These days, most Citrus varieties are grafted on to the closely related Poncirus (P. trifoliata), both being members of the Rutaceae plant family. The trifoliate orange, as it's known, gives good vigour and resistance to the defoliating and debilitating phytophthora disease that citrus are susceptible to in heavy, clay soils, especially those found around Nelson.
Quince, Cydonia, is another tree often grafted on to another, closely related species of Pyrus (pear) rootstock, both being members of the Rosaceae family.
It's a tangled web of relationships that can also be further complicated by the use of an inter-stock, or a third plant, grafted in between the scion and rootstock, to overcome incompatibility where it may exist between the top and rootstock.
Then you can have multiple varieties of, say apples, grafted on to the one rootstock to produce what could be called a "family tree", providing different varieties of apples ripening through the season, to keep the whole family happy. Although commonly sold as double or triple grafted trees, there's no reason why you can't graft a number of varieties on a single rootstock. Sometimes orchards will also have an essential required pollinator variety grafted on to desired fruiting trees at various intervals, to ensure good fruit set.
While trees are usually grafted using what's known as the whip and tongue method of meshing interlocking cuts in the rootstock and scion shoots, supertoms are commonly made using an "approach graft", where both shoots are sliced lengthwise to expose the cambium tissue, then bound together while still growing. Once the graft has taken, the unwanted top and root growth is then cut off, allowing the desired scion and rootstock to carry on. It's not rocket science, but it does require a careful touch, and needs the science to work for the graft to be successful.
Meanwhile, thinking of a suitable name for an eggplant-potato combo I think the egg potato, or the Eggpot, could be the next big thing.
- © Fairfax NZ News