imum ▁of ▁ 6 ▁hours ▁during ▁summer ), ▁mo ist , ▁acid ic ▁so ils ▁( ph ▁level ▁of ▁ 6 . 5 ) ▁with ▁a ▁high ▁organ ic ▁and ▁high ▁fert ility ▁content . ▁ ▁For ▁best ▁flow ering , ▁mo ist ure ▁is ▁essential ▁during ▁late ▁aut umn , ▁winter ▁and ▁spring ▁times ▁( Oct ober ▁to ▁May ), ▁when ▁the ▁plant ▁starts ▁to ▁grow ▁new ▁leaves . ▁They ▁can ▁be ▁given ▁a ▁light ▁feed ▁between ▁aut umn ▁and ▁spring , ▁if ▁needed . ▁ ▁It ▁is ▁estimated ▁to ▁be ▁hard y ▁to ▁between ▁US DA ▁Z ones ▁ 3 ▁to ▁ 1 0 . ▁ ▁It ▁can ▁toler ate ▁fro st ▁but ▁will ▁flower ▁poor ly ▁in ▁areas ▁with ▁cool ▁sum mers . ▁ ▁It ▁can ▁be ▁grown ▁in ▁southern ▁UK , ▁in ▁d amp ▁or ▁wet ▁so ils . ▁They ▁can ▁be ▁grown ▁in ▁most ▁of ▁the ▁United ▁States , ▁New ▁Zealand ▁and ▁Australia . ▁They ▁can ▁be ▁used ▁plant ed ▁by ▁the ▁edges ▁of ▁p onds ▁and ▁po ols ▁or ▁they ▁can ▁be ▁plant ed ▁in ▁the ▁water . ▁But ▁they ▁need ▁the ▁rh iz om es ▁to ▁be ▁p inned ▁into ▁the ▁ground ▁to ▁stop ▁them ▁floating ▁away . ▁Also ▁the ▁rh iz om es ▁must ▁be ▁covered ▁in ▁gra vel ▁to ▁stop ▁fish ▁disturb ing ▁the ▁roots . ▁They ▁can ▁also ▁be ▁plant ed ▁in ▁streams , ▁but ▁not ▁heavy ▁flow ing ▁rivers . ▁It ▁can ▁toler ate ▁po ols ▁up ▁to ▁ 6 ▁inches ▁deep