islayhawk wrote:

I have an area of established wild grassland that I would like to plant
with wildflowers. I do not want to cut it back due to other flowers
growing there. area is about 150 square mts. Would I be better planting
with plugs or seeds. How many plugs or how much seed.

You'd do best cutting it all back, just mow at your mower's highest
setting... don't worry about your previous plantings, the perennials
will grow back in spring and mowing will ensure all reseed. Your area
is about 1,400 sq ft, not much to mow.

I have a 4+ acre wildflower meadow (takes me a full day to mow), it
contains one spring fed 20' deep pond and several smaller shallow
vernal ponds, with lots of wildlife. Each fall I rough mow the entire
meadow to keep it healthy, mowing disperses the seed heads and covers
them with mulch... that's the time to add seeds (move some mowed
debris, scratch soil lightly, scatter some seed, then cover with mowed
mulch). High quality wildflower seed blends can be pricey so strew
very sparingly, don't need much anyway... best to plant several small
areas (naturalized) rather than all in one heap. I plant daffodils
and irises, they flower first and deer won't eat those. I mow about
six inches high so as not to harm the small critters (frogs, lizards,
mice, snakes, etc.), most scatter ahead from the mower vibration.
Mowing also prevents birds/deer from scouring all the seed heads
clean. Be very careful about what seeds you plant, you don't want to
plant any invasives.
Meadow:
http://i41.tinypic.com/m8cwwl.jpg
Rough mowing, I maintain walking paths all around the cicumference:
http://i40.tinypic.com/20s7j2b.jpg
Pond:
http://i40.tinypic.com/2vipbwh.jpg