Background and aims: Phytotoxicity following addition of hydrothermal-carbonised waste amendments (hydrochar) to soils is primarily attributed to toxic-organic compounds formed in hydrochars during hydrothermal carbonisation (HTC). However, factors influencing toxin formation in hydrochar and subsequent phytotoxicity have not been elucidated. Here, we investigated the effects of hydrochar feedstock and HTC temperature on phytotoxicity. Methods: Hydrochars from sawdust, rice straw, chicken manure, paunch-hair, pig manure, biosolids and digestate, produced at three HTC temperatures (170, 200 and 260 °C), were assessed for phytotoxicity using plant-bioassays, spectroscopy and wet-chemistry. Results: Hydrochar had no effect on seed germination, but reduced (30 to 50%) or had no significant effect on wheat growth under limited nutrient supply. Importantly, under luxury-nutrient supply, hydrochars (170 and 200 °C) that reduced growth in limited-nutrient conditions had no significant effect, and only hydrochars produced at 260 °C consistently reduced (20 to 30%) growth. Elemental-analysis and fourier transform infrared spectra indicated an increase in potential toxic functional groups in hydrochars produced at high temperature (260 °C). This suggested that phytotoxicity was due to toxic organic compounds, and occurred at high temperature. Conversely, at low temperature (170 to 200 °C), apparent phytotoxicity in nutrient-limited conditions was not due to hydrochar toxins, but nutrient deficiency exacerbated by hydrochar-induced nutrient immobilisation. Feedstock-type had no significant effect on phytotoxicity. Conclusion: Findings provide new understanding of hydrochar-induced phytotoxicity. Fundamentally, hydrochars (170 to 200 °C) are potential soil-amendments, but nutrition regimes to offset nutrient-drawdown need consideration. Research to mitigate toxicity in hydrochar-260 °C is warranted.