North Yilliminning reserve is 22km from Narrogin on the Narrogin-Kulin Road. This road roughly follows the ancient Binneringie dyke, which has been the catchment divide between the Hotham and Blackwood rivers for hundreds of millions of years. Coming from Narrogin prepare to turn off into a rest bay under trees on the right, soon after passing Taylor Road. Armstrong road marks the eastern side of the reserve.
A breakaway following the highway on the northern edge of the reserve is a contact zone between dolerite to the north (fertile york gum farm land), and granite to the south. The breakaway has brown mallet and red morrel woodland with sparse understorey at its base and patchy lateritic woodland at the top.
The reserve is underlain by granite mostly hidden under sandy, gravelly, and ironstone soils. Most of the reserve is a gently sloping upland with old and infertile soils. Topsoils on the ridges are stony and shallow, with white sandy topsoil on lower slopes and valleys. An old vegetation map of the reserve has colourful descriptions, such as 'poor wandoo' and 'useless plants and dryandra'.
There is attractive bushwalking country. Being a sandy block, there is a multitude of cowslip orchids. Litttle pink fairy orchids also occur with sugar, snail, greenhood, jug, and donkey orchids, and a patch of purple enamel orchids. Unlike many surrounding reserves there are very few spider orchids.
Orchid hybrids tend to be more common in spots where there are lots of one parent and fewer of another. Here cowslip orchids greatly outnumber little pink fairy orchids, and normally flower for much longer. I noticed that hybrids are much more common on shallow sandy gravel slopes, where soils hold less water than deep sands and sands over clay. I think that hybidisation is enhanced by periodic water shortages. which reduce the flowering period in these soils. In drier seasons, the flowering period for the cowslips on these soils is shortened to that of earlier flowering little pink fairy orchids. Here pollinators are more likely to visit both species together and create hybids.
Hybrids are generally sterile, but expand asexually from root tubers into clumps.
You can see the amazing hybrid variation in this reserve in this blog.
The best time to see the hybrids is about the third week of August. To see them, turn into the parking bay with a picnic table and continue up a dirt track going south. Soon after you go over a low ridge (about 200 metres from the picnic table) park in a dirt bay on the left (east), which is marked by a stone cairn and a rusty tin. Look on either side of the track here. There are more hybrids on the eastern side, but more attractive wildflowers and bush on the west. The track ends at a dead end a bit further on. It is a good spot to wander through the open woodland to the south and west where there are many cowslip orchids and some more hybrids.
The eastern edge of the reserve has pockets of deep white sandy soil, which supports remnants of more westerly occurring plants. Of particular note are Jarrah, Candlestick banksia Banksia attenuata, and Christmas tree Nuytsia florabunda. The latter two may be declining. The understorey contains some lovely plants;- Purple enamel orchids (October), stilit walker triggerplant Stylidium araeophyllum, Eremea pauciflora, and Calytrix flava.
There was a wonderful flower show on this sandy patch this year, but as most of these plant flower in late October, November, there would be few late spring flowers in dry seasons.