Your sacred space will be used for meditation, prayer, sending healing energies and writing in your journal. This space is for your introspective process, and it is important that it is undisturbed by outside influences.
I have to admit I am not a nature meditator, unless it's at a beach and the lull of the waves transports me. Generally outside, I find I end up focusing more on the bugs or ground beneath me.
It is exquisite to reserve a place exclusively for peace, and peace has a flavor that intensifies when concentrated. However, it's not necessary to depend on that space to meditate: You can take your practice wherever you go once it is established.