Planted this Stunning Purple Flax Today

Planted three blueberry bushes, two hebe’s and this dramatic looking flax today. I also painted our new wooden letter box in the colours of our house giving it an ironsand roof and the rest in blood red ‘lonestar’, painted the letter box post itself too. 

About six weeks ago I replaced a libertia which had been rotting at the stem, and the leaves were turning yellow, but now the same is happening to the libertia I replaced it with! So I’m thinking the issue must be in the soil? So I’m reluctant to replace that one until I fix the problem - any ideas/suggestions anyone?