In 2009, Adrian Y. Wartono observed a putative cross between N. dubia and N. jamban in an area where these two species grew with N. lingulata and N. rhombicaulis.
Nepenthes rhombicaulis is known from a number of peaks in the Indonesian province of North Sumatra, particularly around Lake Toba.
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Nepenthes rhombicaulis was first collected by Shigeo Kurata on March 29, 1972, on Mount Pangulubao at an altitude of between 1700 and 1900 m above sea level.
These are N. gymnamphora × N. spectabilis, N. mikei × N. spectabilis, N. rhombicaulis × N. spectabilis, and N. spectabilis × N. tobaica.
Nepenthes clipeata | Nepenthes bokorensis | Nepenthes | Nepenthes rajah | Nepenthes mantalingajanensis | Nepenthes klossii | Nepenthes faizaliana | Nepenthes epiphytica | Nepenthes abgracilis | Nepenthes rigidifolia | Nepenthes mira | Nepenthes khasiana | Nepenthes kerrii | Nepenthes deaniana | Nepenthes cid | Nepenthes aristolochioides | Nepenthes viridis | Nepenthes ovata | Nepenthes mirabilis | Nepenthes spathulata | Nepenthes rhombicaulis | Nepenthes pitopangii | Nepenthes × kinabaluensis | Nepenthes holdenii | Nepenthes × harryana | Nepenthes dubia | Nepenthes burbidgeae | Nepenthes bellii | A skeletal revision of ''Nepenthes'' (Nepenthaceae) | ''Nepenthes smilesii'' from Kirirom National Park |