According to [1], this is how to identify Hakea sericea:
Leaves: evergreen, in the shape of a needle of 0,5-1,5 mm diameter, very robust(!), of 4-8 cm, extremely sharp, dark green to greyish-green.
[Presumably this description applies only to adult plants.]
(This description might be wrong: the features indicated with (!) actually seem match Hakea decurrens [5] more closely than Hakea sericea [4].)
Mr. Arthur D. Chapman kindly compared the descriptions of Hakea sericea and Hakea decurrens ssp. physocarpa from several sources and organized the information in several comparison tables. The resulting work can be consulted here: https://drive.google.com/file/d/1367CE1j6kS6J4Obqsr5fDdgz2yW9-xgA/view
Some months ago I also tried to contrast the descriptions of both species, from a couple of sources, and compared them with my own observations. In the following,
Pistil / gynoecium
HS: (-)(4.5–) 5–7 (–7.5) mm long [6]
HS: gynoecium (-)4–7.5 mm long [7]
HD: 8.5–12(+) mm long [6]
HD: gynoecium 9–12(+) mm long [7]
HDp: pistil (+)8.5–12.2 mm long [8]
Pedicels (!!)
HS: sparsely(??) white-pubescent(+) [4]
HS: pedicel villous(??) to hirsute(??) [6]
HS: pedicels villous(??), hairs white [7]
HD: pubescent(??) [5]
HD: appressed(??) pubescent(??) [6]
HD: tomentose(??) or appressed(??) pubescent(??), hairs white(+) and/or rust-brown(-) [7]
HDp: pedicels 1.2–4.8(??) mm long [8]
Perianth
HS: 4–5(??) mm long, white(+), glabrous(+) [4]
HS: perianth (-)2.5–4.7 mm long [7]
HS: perianth white(+) [6]
HS: flowers white(+), pink in bud(+) [7]
HD: 4–7(+) mm long, white(+) or sometimes tinged pink(?), glabrous(+) [5]
HD: 4.2–7.2(+) mm long [7]
HD: perianth white(+) or pink(-) [6]
HDp: perianth 4.2–7.2(+) mm long, glabrous(+) [8]
HDp: white(+) to pink(-) flowers [8]
Rachis
HD: knob-like(?!), [up] to 3(??) mm long [5]
HDp: rachis simple(?!), 0.5–2.8(??) mm long, with tomentose(??) or appressed(??) white and/or ferruginous (-) hairs, extending onto pedicels(??) [8]
Young branches / branchlets (!!)
HS: young branches white(+)-pubescent(?!), glabrescent(+) [4]
HS: branchlets persistently woolly(-) tomentose(-) [7]
HD: new growth glabrous to sparsely(+) or densely(-) hairy [5]
HD: branchlets quickly glabrescent(+) or persistently(-) and densely(-) tomentose [7]
HDp: branchlets sparsely(+) to densely(-) appressed-sericeous, quickly glabrescent(+) or persistent to flowering [8]
Leaves (!!)
HS: leaves ± at right angles(+) to stem [4]
[Presumably this applies only to old branches in adult plants.]
HS: leaves spreading widely(+) to narrowly(+) angled to stem, flexible(+) or rigid(+) [6]
[In Lousã, all plants less than 3 years old - especially those growing in shaded areas - have flexible branclets and very flexible leaves narrowly angled to stem. This also applies to new branches in slightly older plants, perhaps up to 1.5m high.]
HD: leaves spreading widely(?) from stem [5]
HD: leaves spreading widely(?) from stem, rigid(?) [6]
HDp: leaves widely spreading, grooved below(?!) to varying extents, 1.5–8(+) cm long, 0.7–1.6(+) mm wide, rapidly glabrescent(??)
[I never saw any hairy leaves, only hairy branchlets, therefore "glabrescent leaves" doesn't seem to apply.]
Follicle
"Decurrens has smoother(+) fruit than sericea which is tubercular(?!) to deeply wrinkled(-)." [10]
"Fruit on decurrens is narrower(-) and has a prominent beak(+) with two horns(+) but are often eroded. Sericea is broadly ovoid(+) and horns are often obscure(-)." [10]
HD: ± ovoid, 18–35(//) mm long, 14–36(//) mm wide, covered in discrete warts(+); beak prominent(?!); horns present(+) [5]
HDp: fruit 2.1–3.2(//) cm long, (-)1.3–2.5 cm wide, finely or coarsely tuberculate(+), obliquely(-) ovate to broadly ovate(-) [8]
HDp: beak small(+) to moderately large, sparsely pustulate or smooth [8]
HDp: horns (+)1–5 mm long [8]
Lignotuber
HD: lignotuber present(-) [5]
HD: lignotuberous(-) [6]
Mucro (!!)
HD: 1mm long [5]
HDp: apex porrect(?!), with mucro 1–3.5(+) mm long [8]
Overall aspect:
"Sericea appears much denser(+) than decurrens." [10]
HD: spreading shrub(//) to small tree 0.3–5(-) m high [5]
HDp: small tree or shrub, 0.8–5(-) m tall [8]
Distribution
HD: (...) often grows in sandy(-) or rocky(+) situations [5]
HDp: found in eucalypt(+) forest, damp heath(+) or dry scrubland in hilly(+) areas in sand(-), clay(-), granite(-), basalt(-) or sandstone(?!), from sea-level to 300(?) m [8]
[Our spiky Hakea trees are present at least 500 meters above sea level.]
HDp: (...) also in Portugal [9]
GLOSSARY:
REFERENCES:
[1] https://invasoras.pt/en/invasive-plant/hakea-sericea
[2] https://en.wikipedia.org/wiki/Glossary_of_botanical_terms
[3] https://www.inaturalist.org/observations/58692770
[4] https://plantnet.rbgsyd.nsw.gov.au/cgi-bin/NSWfl.pl?page=nswfl&lvl=sp&name=Hakea~sericea
[5] https://plantnet.rbgsyd.nsw.gov.au/cgi-bin/NSWfl.pl?page=nswfl&lvl=sp&name=Hakea~decurrens
[6] https://vicflora.rbg.vic.gov.au/flora/key/2411
[7] https://plantnet.rbgsyd.nsw.gov.au/cgi-bin/NSWfl.pl?page=nswfl&lvl=gn&name=Hakea
[8] http://www.flora.sa.gov.au/efsa/lucid/Hakea/key/Australian%20Hakea%20species/Media/Html/Hakea_decurrens_ssp._physocarpa.htm
[9] https://plantnet.rbgsyd.nsw.gov.au/cgi-bin/NSWfl.pl?page=nswfl&lvl=in&name=Hakea~decurrens+subsp.~physocarpa
[10] https://www.inaturalist.org/observations/59023578#activity_comment_5413253
@belmontmargie: another person, with a lot of plant knowledge, says decurrens has smoother fruit than sericea which is tubercular to deeply wrinkled. Sericea appears much denser than decurrens. leaves in decurrens are widely spreading as in your photo. Fruit on decurrens is narrower and has a prominent beak with two horns but are often eroded. Sericea is broadly ovoid and horns are often obscure.
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