N-fix 6: Senna hebecarpa

One of the few hardy Cassia/Senna’s, Senna hebecarpa.

The method of finding hardy nitrogen fixers I used to comprise my list is pretty simple: choose a genus of nitrogen fixers (I say genus because there are some weirdos that can’t be found from the higher orders and phylum) and look through the whole genus. Often, there will be some member that is hardy in your area.

I consider one of the greatest prizes I found this way to be Senna/Cassia hebecarpa.

Cassia is a genus almost exclusively tropical, and mostly trees, which are gorgeous.

Cassia alota, commonly called “Candlstick Cassia” for its looks. Can’t grow it. Click for source.

Like a lot of the non-hardy N fixers cassia are woody yet legumes and utilize Rhizobium to fix their usually medium level nitrogen.

S. hebecarpa, on the other hand, is herbaceous, and clump forming. Like lespedeza it grows up over the summer and dies back to the ground in winter. Although senna usually stops around 4 ft.

A more established clump of S. hebecarpa. I can’t wait until mine looks like this! Click for source.

This change in height allows for some niche-in-time filling because it makes sunlight and mulch abundant in spring when most perennial vegetables are growing fastest, but in summer and fall covers everything up to protect from wind and make a buffer against quick changes in temperature extending the warm season.

Another plus senna has over lespedeza is its taproot rather than fibrous shallow root system. Senna is native to the eastern US prairies, which were created and partially maintained by droughts that kept the water table below the reach of tree roots. So it makes sense senna would go deep to get its sustenance, leaving the upper ground for shallow rooted plants to play in, so plant away!

I haven’t bothered starting this from seed yet since I found decently priced second year roots available from Prairie Moon Nursery.

The happiest of my three S. hebecarpa this year. It barely bloomed but made a nice four feet of growth.

Companion Plants sells first year seedlings, and lots of ornamental greenhouses sell it because it’s really quite pretty with it’s yellow flowers characteristic of the genus.

Senna marilandica is different from S. hebecarpa only in how its pods break open in fall. So if you have a hard time finding the one the other is an excellent stand in.

Another characteristic I like about this plant is its thigmonasty -the tendency to fold it’s leaves when exposed to touch, or when the sun goes away for night or before a storm (technically photonasty), or when it’s overly hot (thermonasty). Amorpha and several other plants do the same, but senna with its larger leaves is more notable. It’s caused by electrical sensors in the plant modifying the potassium in the leaves causing an efflux of water from the cells, reducing turgor pressure, and thus reducing leaf rigidity.

Being so ornamental and interesting to watch I’ve placed these in very visible spots in the food forest. Of course, I’d make it visible if only to be reminded I have a Cassia in my northern food forest as it seems I’ve got the collector’s bug.



    1. When I was younger that was definitely a characteristic that fascinated me. I’d touch just to make the leaves fold up and impatiently wait for the leaves to open up so I could do it again.
      Glad to know the mechanics of it now.

      P.S. I should also mention that was with sensitive plant’s thigmonasty, not Senna’s. Senna is much slower to respond and not as dramatic. Senna’s photonasty of folding its leaves at night is much more marked.


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