Tree tomato, also called tamarillo, is a fascinating fruit species that mostly predominates in the highland tropics. Years ago my interest was piqued by this fascinating South American species, especially after reading about it in the fantastic book Lost Crops of the Incas. What’s not to love about the idea of a tomato that grows on trees? After reading more about its climatic preferences, clumsily killing a few plants and having discouraging conversations with a few people this plant mostly fell off my radar as something that would not be an option for growing here in steamy Central Florida. That was until a few months ago when I was visiting a tropical fruit farm on Pine Island and just happened to look up and notice a fruit that caught my attention. Incredibly, after years of ignoring this plant, I had been researching it just a few days before this particular excursion. There above me was an established tree tomato plant, with one solitary fruit hanging from it. The grower informed me that a few months previous to my visit there were heavy loads of fruit that were threatening to shear limbs off with their weight. I feel really lucky I was able to see and to collect that one remaining fruit to retrieve seeds for my future tree tomato growing efforts.
What got me really excited was the fact that this tree was growing and fruiting on a property with all kinds of tropical fruit such as jackfruit and even a young durian. This is a piece of land that has a higher average temperature than where I live. This disproved my theory that it is impossible to grow tree tomatoes in Florida because of the hot, humid summer. What allowed this tree tomato to thrive was the fact that is was grafted onto tree potato rootstock (Solanum macranthum). After seeing this I determined that although Florida certainly isn’t an ideal climate for tree tomatoes, most people that have failed growing it probably had plants that succumbed to root-knot nematodes rather than the Florida heat alone. Nematodes are the bane of many kinds of crops, but the Solanaceae family, in particular, gets hit hard by this pest. Common crops like tomatoes and eggplants succumb easily, but I have had this same problem with other related crops such as cocona and naranjilla. In fact, this barrier to successful tree tomatoes seems so obvious that I can’t believe it didn’t occur to me sooner. The grower also had the tree tomato growing in dappled shade. The tree tomato scion was grafted onto a shoot emerging from the base of a quite large Solanum macranthum, and much of the tree tomato was protected from the intense sun by the mother’s canopy. Also of interest was grafted, and fruiting cocona (Solanum sessiliflorum) emerging from a different shoot of the same tree.
Grafting is a powerful horticultural technique for cultivating fruit crops. Generally grafting is used to clone a desirable variety of fruit and the main focus is on the top of the tree. What is sometimes overlooked is the great capacity to grow otherwise un-growable crops that face some sort of issue with their roots. Root problems can include pH issues, nematodes, Phytophtora/root diseases, salinity and probably all kinds of other things. Thankfully, many plants are compatible with other species that are closely related. Usually, graft compatibility is within the same genus but sometimes inter-generic grafts are possible. Tree tomato and potato tree are currently classified under different genera so this is one of those less common cases. Solanum macranthum is an ornamental flowering plant that happens to be vigorous and totally unaffected by root-knot nematodes. I will also be trying to graft to Solanum torvum, which I expect to work fine. In the past, I have grafted eggplants to this species with success, which I actually filmed a video about. That video can be watched here. Solanum torvum is also grown as a food crop, with the small, bitter eggplants being eaten in Southeast Asia, the Caribbean and Africa.
Grafting may seem intimidating and there is a learning curve, but the only way to learn is to jump in and try. Note that this paragraph is describing soft-tissued vegetable grafting, which is quite different and in some ways easier than grafting woody plants. In this case, I started the rootstock seed and tree tomato seed the same day. These grow about the same speed, but in other cases you can stage the seeds to start on different days so at grafting time the seedlings are the same size. I prepped the scion by removing all the largest leaves and leaving only the newest growth and the second newest leaf, which I cut in half. Removing the leaves may seem counterintuitive, but leaves transpire water away and dry out the scion, which will kill it. I did a cleft graft on this one, but side veneer or modified cleft graft/bark graft would probably work better with larger plants. What is nice about grafting vegetables is that a large percentage of the cross-section of the stem tissue is cambium so one does not need to be especially precise about lining up the scion to the rootstock. I used a small piece of Parafilm to seal up with graft and pull the union together tighter. I found this to be a little tricky with such soft plant material and small diameter stems. After grafting, I put 4 sticks in the corners of the pots to support a produce bag, which makes a humidity dome. The grafted plants will be in deep shade in the humidity dome while the grafts heal, then I will gently transition into the sun and slowly remove the bag to acclimate the plant to a lower humidity situation. It is important that the humidity dome is never in full sun or the plants will cook. There are plenty of great manuals and videos available online about vegetable grafting, which is basically the same process as grafting tree tomatoes.
I found in research that tree tomato is grown commercially on a special rootstock in Kenya. The rootstock used is locally called bug weed and as far as I can find, is Solanum mauritianum. This rootstock is also supposed to impart drought tolerance to the tree tomato plants. This species is reported to be poisonous, which brings up a concern. Can poisonous rootstocks impart toxicity into otherwise edible Solanaceae species? I asked the Pine Island grower about this because Solanum macranthum is also presumably toxic. He insisted that he has eaten a number of Solanum fruits grafted to S. macranthum rootstock and they are entirely edible. The Kenyan experience with S. mauritianum seems to be the same but some caution should still probably be employed working with new toxic Solanum rootstocks. One benefit of Solanum torvum is that this species itself makes edible fruit so there isn’t any worry of traveling toxins.
Tree tomato is a plant that excites me a lot. As one friend pointed out, here in Florida we have all kinds of alternative perennial crops to standard grocery store vegetables. For squash, we have chayote. For cucumbers – tindora. For spinach – a whole host of perennial leafy green plants such as chaya. There are several great tropical root crops that can be grown to substitute potatoes. But what about a perennial tomato? Tomato is really a staple item in virtually all cuisines around the world. I can’t imagine food without tomatoes which is why I go through a lot of labor to grow them in a special greenhouse. To be clear, tree tomato does not taste exactly like a tomato. To me the flavor is something like a mild passion fruit mixed with a tomato, mixed with something all of its own. But there is a distinctive tomato flavor in there, and I imagine with some creativity it can take the place of a tomato for some dishes. Otherwise it can be enjoyed as a nice fruit on its own. I certainly don’t expect to stop growing and eating tomatoes but it would be wonderful to have a close approximation that grows on a robust tree-like plant.
To grow any species that is out of its comfort zone, one simply needs to look at the major factors that make the species challenging and then evaluate ways to adjust conditions to make life more hospitable for the plants. In the case of tree tomato, the biggest challenges seem to be root-knot nematodes and extreme heat. The plants are also probably quite cold-sensitive, which will require them to be protected. My overall strategy for growing this fruit in Central Florida will be four-pronged.
1. All plants I put out will be grafted onto S. macranthum or S. torvum. This will eliminate the root-knot nematode issue.
2. I will experiment with different varieties. There’s a good chance that some strains are more adapted to hot, humid Florida conditions than others. I’m growing out a few different varieties, which I will evaluate for their performance in this climate as well as fruit quality. I’m also growing out seedlings of another tree tomato species, known as the forest tamarillo – Cyphomandra sibundoyensis. There are quite a few species of Cyphomandra that may have decent fruit that are worth experimenting with.
3. I will plant in dappled shade to give plants the lower ambient temperatures that they prefer.
4. I will plant most of my stock into areas of the farm that receive overhead water protection on freeze nights.
This post is only the germ of this project. I’m sharing my aspirations for tree tomato and my first steps towards growing them. I plan to share future progress reports about the success of my grafts and how the plants ultimately grow and fruit. Of course, the whole thing could completely fail for unforeseen reasons, but I feel really optimistic that I will have some success after seeing a large fruiting specimen on Pine Island. I would be pleased to get other people excited about working with this interesting fruit species in the mostly uncharted tree tomato territory of Florida.
