Hi, it is best to fertilize only with low phosphorous fertilizers. We can buy them here as 'native' ferts (many Aus. plants will not tolerate phosphorous - our soils are naturally deficient in it). If you cant buy low phosphorous fertilizer use blood and bone, watered in very well. You can also increase growth and vigor with fish emulsion foliar feed and a seaweed tonic.
They are extremely slow growing, one for the grandkids!!
Hope this helps - good luck.

great great great grand-kids might be able to make a bonsai out of it.
:-)

What sort of grasstree? Xanthorrhoea?
The idea that Xanthorrhoeas are incredibly slow-growing - one inch per century - came from a specimen studied by botanists in the Edinburgh botanic gardens - hardly an ideal climate for them.
Some grasstrees will grow stems taller than you, within a century - hardly fast-growing, but better than the reputation suggets. In a garden, some species can form a nice plant with a trunk a foot or so high, and a pom-pom of leaves more than a metre across in less than 20 years.
(Lawn fertilisers are usually low in phosphorus)
Trish