Inspired by a private conversation with @stephane_rave, I wanted to make this thread to explore one of my favorite spice crops: The citrusy, mouth-prickling and numbing fruit of the genus Zanthoxylum.
You might have encountered Sichuan pepper (mostly Z. simulans and Z. bungeanum), Sansho pepper (Z. piperitum) and Timut pepper (Z. armatum). Other species used for culinary purposes include Z. schinifolium (Dog Sansho), Z. rhetsa, Z. avicennae, Z. acanthopodium. The genus contains a mind-boggling number of species (probably around 250) and variations. And the taxonomy gets complicated because the spice has different names in different regions and sometimes those names cover several close species. In other words, probably lots of micro-species capable of hybridizing with each other.
Red fruits from Zanthoxylum bungeanum
Taxonomists place the genus in its own subfamily (Zanthoxyoideae) in the citrus family (Rutaceae). I sometimes tell my chef students that they’re eating the world’s smallest citrus fruit, when we sample fresh Sichuan pepper.
Center of origin. Most species with a culinary tradition come from the region around the Himalayas with many other species in other places of Asia. North America has some important species too that have been used by indigenous peoples too.
Like other plants in this family, the leaves contain glands with aromatic compounds and the leaves of Zanthoxylum species have a wonderful flavor similar to the fruit. I prefer to pick the leaves when they are juts leafing out, contain very little fibre and have a succulent mouthfeel.
Gernot Katzer has a good overview of the chemical composition of the main aromatic compounds. Most species produce alkamides with the characteristic sharp, fresh smell and taste. You can find similar alkamides in the flowers of buzz buttons (Acmella oleracea) and I often challenge chef students to name the connection when we taste them (as soon as someone says it, everyone tastes the connection which shows how our sensory experience has everything to do with using our whole brain!). The characteristic aroma come from aromatic oils, usually citrusy terpenoids, but the precise composition vary immensely between the species - giving rise to lots of flavor variations. Most abundant oils are geranyl acetate as well as the monoterpene limonene and the aldehyde ctronellal. Other compounds include ß-myrcene, ß-ocimene, linalool, citronellol, sabinene, geraniole. If you look up those compounds, you’ll see how they often appear in other well-known spice and perfume plants like roses, geranium, citrus zest, juniper, umbellifers, hops and many more.
Stéphane came up with the great idea that we try to landrace these plants by exploring the genus as whole, looking for possible hybridizations. We will exchange genetic resources and I invite other people to join this project. We’re both in the EU, so collaborations will come more easily here, but I don’t see a reason to not work together across borders.
Hardiness. So far, I’ve based my growing on getting as many species as possible to try out and test for hardiness (in two different sites at USDA zone 7 and zone 9). USDA hardiness zones misrepresent the challenges in my region. We have relatively mild winters, but usually not enough heat in the summer to harden off the wood. In a climate with hotter summers, you might have luck growing some of the same specie with colder winters, because the wood hardens off sufficiently.
Many species seem to tolerate or even thrive as an understorey tree, ie. partly shaded. I believe the key here is shelter and it wouldn’t mind the sun. The largest Zanthoxylum tree in my country I’ve found in the botanical garden as a 4 meter high multi-stemmed tree with open canopy in a partly shaded spot behind taller trees.
At the coldest site, I have trialed the following species:
Z. armatum
Z. bungeanum
Z. clava-herculis
Z. piperitum
Z. schinifolium
Z. simulans
The two most famous species Z. piperitum and Z. simulans have consistently died on me in our winters (tried several times). Zanthoxylum coreanum survived a winter with severe die-back and then died the next winter. Zanthoxylum schinifolium I off to the side in what ended up providing more shelter and it has survived relatively well. Zanthoxylum bungeanum stands out as the most hardy species in my garden: Consistently overwinters with no die-back and has reached a size now that I get a good crop (and seed!).
I’ve planted some of the other species in a city garden (zone 9) that I only get to visit once a year and I lost the record of which species I put there. Most species seem to go much better in sheltered spots in the city. My next phase is to test more species in a new publicly accessible urban garden (zone 9). I’ve planted Z. simulans so far and will add more.
As with many trees that have hardiness difficulties, you can sometimes succeed by overwintered the small trees frost-free for a year or two. When the plant gets more mature, it will be able to grow faster and harden off the wood more easily. So you can plant it out on year 2 or 3 for example.
Propagation by seed. I find most species very difficult to germinate if seed has gone into deep dormancy, which seem to happen as soon as it dries out. I recommend getting fresh seed if possible.
Pruning. Most species never get taller than 3-5 meters and can easily fit in somewhere (mind the thorns though). If you’re lucky enough that they grow well for you and want to keep the trees smaller, they easily tolerate pruning.