Sumpweed (Iva annua) is also sometimes called marshelder and (as you might have already guessed) it likes to grow in places that are a bit swampy. It is a prolific seed producer and very good at forming dense stands and out-competing everything else. It will be hard to eradicate once you grow it somewhere. It is also allergenic for some (it’s related to ragweed). I recommend you give sumpweed its own space away from other garden plants. If you’ve got an area that is a little too wet for comfort in the spring, grow it there.
WARNING: CONSIDER BEFORE PLANTING
Planting
Sumpweed should be grown somewhere with consistently moist soil throughout the spring. In the summer and fall, it can tolerate drying out, but it won’t germinate in a dry spot. I was able to achieve ~70% germination after a stratification of 6-8 weeks in moist soil at 4 degrees C (a normal fridge temp).
Stratification can be accomplished in a few ways:
- Plant seeds in the fall to mimic natural dispersal.
- Plant seeds in the late winter, as soon as the soil is thawed.
- Mix seeds into wet top soil, store this mess in a refrigerator, then spread it over your garden bed. I had good success in 2020 in St. Louis with planting pre-stratified seeds out in mid-April. If you’re further south or north, adjust accordingly.
- I’ll be adding a guide on stratification methods for controlled experiments that I’ve used soon!
If you get dry weather after planting your sumpweed seeds, make sure you water them every day. I had almost no germination in my 2018 experiments in Ithaca because my field was on top of a hill and we didn’t get much rain. Sumpweed does fine with being transplanted, though. That year, we moved plants from the greenhouse outside in June and they did great.
My preliminary data suggest that you should cover the seeds in about 1/4 inch of soil. In 2020, I did an experiment comparing three different treatments to control plots. All but the “deep planting” treatment were planted under just a sprinkling of soil. About 1 month after I planted these seeds, the plants in the plots where I had planted the seeds more deeply (1/4 inches of soil) were bigger and approximately the same density as the other plots. Unfortunately, I never actually counted how many plants germinated in each plot before this experiment got eaten by deer, but here are the plant heights in late May:
This data also suggests that you should also add some fertilizer to your soil. I used cow manure in this experiment, but compost would probably work just as well. This makes sense since (I’ve argued) that sumpweed co-evolved with bison, is dispersed in their dung, and loves to grow in their nitrogen enriched wallows. Plant your sumpweed with some bison for best results!
And one more thing — protect your sumpweed with a nice tall fence if there are deer around. It may surprise you once you see how smelly and hairy sumpweed leaves are, but I’ve discovered that at least two creatures besides people like to eat sumpweed — deer and bison.
Tending
You may want to thin out your sumpweed in early summer if you are trying to maximize the amount of seed you get. In a preliminary experiment we ran on this in 2019, we had two treatments, high density (~15 plants/ sq meter) and low density (~50 plants/sq meter). The low density treatment resulted in plants with higher yields on average — but since there were more plants in the high density treatment, it was higher yielding/area. However, if left untended, sumpweed can reach much higher densities than ~50 plants/sq meter. Liz Horton found that the sumpweed she grew in the Plum Bayou Garden at Toltec Mounds, near Little Rock, Arkansas, grew so thick that it actually started rotting — probably not good for any plant. I have more data on density/yield from free-living plants in Oklahoma, which I will add soon.
Harvesting
There is a fairly long time window for harvesting sumpweed, between late September and late October. It doesn’t seem to matter matter much if you harvest seeds of plants that are still a little juicy or wait til they are completely dead in terms of seed viability. However, it does matter in terms of how you process and treat the seeds for the next generation (see below). The timing of sumpweed senescence (death) seems to be related to plant density — low density plants or plants growing on their own will be ready to harvest earlier than high density plants. Ex. In Illinois in 2019, I harvested the low density plants between October 2 and October 23, and the high density plants between October 23 and October 28, as they senesced. That same year, I harvested wild plants in northern Oklahoma between September 29 and October 20. Here’s what the plants looked like on these dates, respectively:
I’ve harvested sumpweed as you see above, by hand stripping seeds in the field into paper bags or fabric pop-up garbage cans (sumpweed can produce a ton of seed). If I have a lot of sumpweed and the weather isn’t this idyllic, then I harvest by cutting off the entire plant at the base of the stem with a pair of shears. I stuff the plant or plants into paper grocery bags and let them dry in a cool, dark place for a week or so. Then I hand strip the seeds, and dry them further on trays lined with chiffon that I made to fit a pizza dough rack (see the goosefoot guide for a picture of this contraption). Either way, wear gloves. Many people get a rash from handling fresh sumpweed.
Processing and cooking
If you choose to harvest sumpweed before it is senesced, you will need to dry it for longer (probably a couple of weeks) before threshing and winnowing. See my guide to threshing and winnowing (coming soon) for instructions about how to proceed from here for the seeds you want to cook. As for rendering these little beasts palatable — this is the million dollar question. We know people ate them because they turn up in ancient poops, but they are extremely bitter. Please do let me know if you make anything tasty with them.
The next generation
Sumpweed will definitely come back in the same place you grew it last year, even if you try to harvest all the seeds. But just in case, make sure to save some seeds. Store them in the freezer if you’re not planning on using them right away.
My experiments in Ithaca suggested that if you harvest sumpweed early, then it is harder to break germination. Seeds I harvested in early October did best with an 8 week cold stratification, whereas seeds I harvested in early November did best with 6 weeks. I’m not sure how this works yet, but be aware of this difference when preparing or planting a second generation.