yellow mustards

Member of mustard family (family Brassicaceae)
dicots (class Magnoliopsida)
flowering plants (subphylum Angiospermae)

This page describes the most common mustard relatives with yellow flowers that are 1+ inch in diameter. Smaller and rarer flowers are not covered here.

ID tips are generally from Jepson eFlora unless otherwise stated.

ID tips marked [IF] from the iNaturalist forum.

ID tips marked [MCW] are from Monterey County Wildflowers.

ID tips marked [HPB] are from Harvard Papers in Botany, June 2021 [PDF].

A basal rosette may be obscured by live or dead vegetation, or the rosette itself may have withered away. The absence of a basal rosette doesn’t seem to be a useful distinguishing characteristic.

The fruit of all species is linear and generally splits into two valves along most of its length, but with an indehiscent terminal segment. Young fruit may be indehiscent throughout[?].

Key features:

fruits appressed to stem or not
fruit shape (the more mature, the better): side view and cross section
plant height
hairy lower stem or not
basal leaves: present as a rosette or not, lobe pattern
cauline leaves: many or few
flower bud: overtopping flowers or not, hairy or not

field mustard
Brassica rapa

fruit not appressed

fruit 3–8 cm long, 2–4 mm wide
terminal segment tapered [MCW], 8–22 mm long

stem 30–100 cm
few or no hairs
basal leaves may be pinnately lobed, with the terminal leaflet being the largest [MCW]
middle and distal cauline leaves sessile, base lobed, generally clasping
open flowers often overtop flower buds
very common
there are other cabbages and mustards (genus Brassica), but these are rare

shortpod mustard
Hirschfeldia incana

fruit appressed to stem

fruit does not have a square cross section; flattened at least in the upper part?
fruit 0.7–1.5 cm long, 1–1.7 mm wide
terminal segment conical, 3–6 mm long, 1–2 seeded

basal leaves in a rosette, pinnately lobed
cauline leaves dentate to pinnately lobed, petioled or sessile, not clasping
most leaves are in the basal rosette; compare to black mustard (Rhamphospermum nigrum)
leaf feels “fuzzy” [basal or cauline?] [IF]
vegetation is dull gray-green; compare to black mustard (Rhamphospermum nigrum) [IF]
leaves have regular lobes with a terminal lobe not much wider than the side lobes

many more basal lobes than black mustard (Rhamphospermum nigrum)

open flowers often overtop flower buds

sepals and buds are hairy [IF]; compare to black mustard (Rhamphospermum nigrum)

very common

black mustard
Rhamphospermum nigrum

fruit appressed to stem, or at least partially so

may be less appressed later in the season [IF]
fruit has 4 raised ribs, giving each a square cross section [IF]
fruit 1–2.5 cm long, 2–3 mm wide
valves have 1 vein each [HPB]
terminal segment 2–5 mm, thin and seedless

stem 30–200 cm

stem hairs unclear: hairy only higher up [MCW] or stiff hairy below [Jepson]

basal leaves are pinnately lobed, more or less toothed [MCW]
cauline leaves are smaller [MCW] with a tapered base, not clasping
stem is much more leafy than shortpod mustard (Hirschfeldia incana) [IF]
vegetation is glossy dark green; compare to shortpod mustard (Hirschfeldia incana)
flower buds often overtop open flowers [IF]

sepals and buds are glabrous [IF]; compare to shortpod mustard (Hirschfeldia incana)

very common, but also commonly IDed by mistake [IF]

charlock
Rhamphospermum arvense

fruit not appressed
fruit linear: 2–4.5 cm long, 2.5–3.5 mm wide, glabrous

valves have 3–7 prominent veins each [HPB]
fruit does not have an obviously square cross section [IF]
terminal segment is conical, inflated around 1 or 2 seeds [HPB] or 0 or 1 seeds [Jepson]

stem 20–100 cm (occasionally much taller)

stem is much more hairy than black mustard (Rhamphospermum nigrum) [IF], but occasionally glabrous [Jepson]

basal leaves not in a rosette
cauline leaves short-petioled, dentate
uncommon

white mustard
Sinapis alba

fruit not appressed
fruit 2–4 cm long, 3–5.5 mm wide

terminal segment flattened, seedless, 6–12 mm long

stem 25–100 cm (ocasionally much taller)
hairy (or rarely glabrous)
basal leaves not in a rosette, pinnately lobed

lateral lobes in 1–3 pairs, dentate/lobed

distal cauline leaves short-petioled, coarsely dentate or entire
rare

4 observed taxons / 1 unobserved taxon / 1 key

Chris’s observations: 24 (4 are research grade)

Locations:

Coyote Lake / Harvey Bear Ranch CP: 4
Coyote Hills RP: 3
Máyyan 'Ooyákma Coyote Ridge OSP: 3
Joseph D. Grant CP: 2
Russian Ridge OSP: 2
Sierra Vista OSP: 2
Calero CP
Coyote Creek Trail
Ed R. Levin CP
Edgewood Park & Natural Preserve
Fremont Older OSP
Los Altos Hills, CA
Mission Peak RP
Saratoga, CA

Months:

Sep.: 2
Oct.: 1
Nov.: 0
Dec.: 1
Jan.: 2
Feb.: 3
Mar.: 5
Apr.: 5
May: 3
Jun.: 1
Jul.: 1

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