The genus Zanthoxylum: Sichuan, Sansho, Timut pepper, Prickly ash and other mouth-numbing citrusy species

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.

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Zanthoxylum americanum is pretty common around here. Called Prickly Ash or Toothache Tree. Usually found as a forest-edge species.

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The genus zanthoxylum is affectively vast…some species are hardy in Europe, some are beautiful if you do not fear thorns, but the taste experience is indeed one of their assets !

I grow :
Zanthoxylum armatum
Zanthoxylum coreanum
Zanthoxylum giraldii
Zanthoxylum simulans
Zanthoxylum schinifolium

most of seedlings are from seeds collect in botanicals gardens .

The difficulty isn’t for me from dormant seeds because I systematically pass the seeds in cold stratification in fridge several months. I have good seedlings, but sometimes seeds do not work well and I feel that hard seeds did not see enough frost once in fresh soil to break their shell.

Then comes the fragile small plant is there the biggest worry of these shrubs begin…are snail magnets and snail that love to destroy them. Up to 1 or 2 years their aromatic bark is too fine and the slugs manage to kill them by eating the cambium.

After that the plants are very robust, they go outside and there nothing even a ruminant would dare to touch them with the thorns…

I cold stratify the seed for 2-3 months as well and get poor germination, probably less than 5%. I even tried scarifying some of the seed with no improvement.

I had already noticed this with Tetradium, some seedlings are very successful others almost zero…So this document gives a clue with the oil content on the surface of the seeds.

use soap to break the oil on seeds!

https://revuesciences-techniquesburkina.org/index.php/sciences_naturelles_et_appliquee/article/view/1002/825

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