Mustard seeds can come from several plants from the Brassicaceae family (cabbage or mustard family). Mustard seeds can be used as spice directly. The seeds are really tiny. The commonly used yellow condiment is made out of the seeds which are ground and mixed with water, vinegar or other liquids.
Mustard is mentioned in relation with Pesach in the Talmud, the Mishna says "Flour may not be put into charoset or into the mustard" (Pesachim 40b). The concern here is that the vinegar in charoset (don’t know what charoset they have, mine doesn’t have vinagar) turns the flour into leaven more quickly. There is some discussion about mustard, but the consensus seems to be that mustard, because it is sharp, retards fermentation. The Rambam (Mishneh Torah, Chametz uMatza 5:19) writes that if one puts flour into mustard, one must eat it immediately (in contrast to flour in charoset, where the mixture must be burned immediately, because it has turned to chametz). Of course, the Talmud is not talking about kitniot on Pesach, this custom developed much later. Their concern is only on the effect of mustard on flour.
Mustard is generally regarded as kitniot. An internet search turns up many interesting reasons why this could be. Mustard seeds are threshed and winnowed in a manner similar to grains (Chabad WMC: Chametz vs. Kitniyos). Mustard seeds grow in a pod like beans (R. Blech: Know Thy Beans with reference to Taz 453:1). Mustard grows in a manner similar to the way the five types of grain grow (Rabbi Michael Taubes on Kitniyos also with reference to Taz). Mustard often grows together with and is harvested and/or stored with the five grains (Dick Israel). Mustard is ground to flour, to make the condiment (Hanny). I have even found a claim that "mustard [… is] not kitniyot, but the custom is to forbid them because they grow in pods like kitniyot" (R. Melamed: Peninei Halakha, The Laws of Pesach, Chapter Nine, Kitniyot). So many reasons probably mean that nobody really knows why, people have just accepted that mustard is kitniot. I personally think that mustard is a case similar to what we discussed in connection with sesame seeds, they are included in the category of "seeds people eat" as defined for Hilchot Kil’ayim (laws of mixtures).
The Rama (Shulchan Aruch Orach Chayim 464:1) states that mustard is kitniot without a real explanation. Other sources given for mustard as kitniot that I have found in my search but not (yet) checked are: Beis Yosef O.C. 453, Rama 453:1 & 464:1, Taz 453:1, Mishnah Berurah 453:4, 7 & 11. So more solid ground than for sunflowers. A source for not regarding mustard as kitniot may be found by Rambam when he is talking about the measurements about a forbidden mixture between mustard seeds and kitniot – which would imply that mustard is not kitniot, otherwise there would be no mixture (Rambam Mishne Tora, Kil’ayim 2:4, credit goes to R. Brofsky).
So where does that leave us? Not far from where we started. Pretty much all kitniot lists contain mustard, but the real reason is unknown.
- Danger of chametz traces: Condiment, possible.
- Danger of confusion with chametz (raw): No.
- Danger of confusion with chametz (processed): No.
- Botanical categorization: Brassicaceae, mustard family.
- Known in 13th century: Yes.
- Verdict: Probably kitniot.
photo (c) Sanjay Acharya
For your amusement, the Gemara on Pesachim 40b (Soncino translation):
R. Kahana said: The controversy is [about putting flour] into mustard; but [if it was put] into haroseth, all agree that it must be burnt immediately. And it was taught likewise: Flour must not be put into haroseth, and if he did put [it], it must be burnt immediately. [If put] into mustard, – R. Meir said: It must be burnt immediately; but the Sages rule: It must be eaten immediately. [The greater strength of mustard retards fermentation, hence the controversy.]
R. Huna the son of Rab Judah said in R. Nahman’s name in Samuel’s name: The halachah is as the words of the Sages. Said R. Nahman b. Isaac to R. Huna the son of Rab Judah: Do you say it in reference to haroseth. or do you say it in reference to mustard? What is the practical difference? asked he. – In respect to R. Kahana’s [dictum] – For R. Kahana said: The controversy is [about putting flour] into mustard; but [if it was put] into haroseth, all agree that it must be burnt immediately. [Do you too accept this, or do you maintain that there is a controversy in respect of haroseth too?] I have not heard it, he replied to him, as if to say, I do not agree with it. R. Ashi said: Logic supports R. Kahana, since Samuel said: The halachah is not as R. Jose. Surely then, since it [vinegar] does not bind, it does indeed cause fermentation? [Which is the reason that flour must not be put into haroseth, since it contains vinegar, and it further follows that if put into it, it is forbidden. Hence when Samuel said that the halachah is as the Sages, that it is permitted, he must have referred to mustard, but not to haroseth.] – No: perhaps it neither binds nor promotes fermentation.