Kniphofia ( red hot poker plant)

[tombstoneteeth]

Does anyone know about how this plant reacts with others? I have it growing on hot, dry clay bank, adjacent to lavenders (i love the orange/purple combo). However, some of the lavenders seem to be failing, and generally those closest to the poker plants. Are they aggressive towards other plants, or could there be a problem with the soil, which was once covered by big old eucalypts and silky oaks?
Thanks!

Are the tree stumps still in the ground?have you seen any small yellow brown toadstools? Ive never known kniphofia to affect adjacent plants. It may be honey fungus affecting lavender roots.

Thank you for the response! I'll certainly check for anything like that under all the foliage.