I first grew this plant in my old garden on the west side of Bainbridge Island. That was when I was first getting into gardening and I planted this poppy near the edge of a steep bank that was held in place by some railroad ties. These ties were about 4 feet high. Anyway, the Romneya liked its spot so well that it sent its roots down under the railroad ties and came up beyond them. This gives an indication that this plant, if it likes its position, can be incredibly invasive.
Knowing the tendencies of this poppy to wander, I have not tried to plant it where I might regret it. However, there used to be a large arbor on the west side of our house that was covered by kiwi vines. The arbor eventually rotted and had to be taken out, and one of the spots that became open after this was sort of a well between the south side of our house and the terrace. This well is about 2 feet lower than the terrace and is probably about 6 feet wide and 15 feet long.
I decided to plant only perennials and not shrubs in this well because I knew that if any work had to be done on our house, any shrub would be damaged. Perennials can take that kind of abuse and come back much better than shrubs can. I also wanted only plants that did not have to be watered and that were tall enough to come up well over the 2 foot height of the well. So, of course Lobelia tupa came to mind as an ideal plant for this area. I planted 7 or 8 of them and they did well, but there were still some gaps. Then, last summer I was at a local retail nursery and they had some very nice looking gallon containers of Romneya coulteri, so I thought these would be ideal to interplant with the Lobelia tupas. The white of the poppy would go well with the orange red of the lobelia and the terrace which surrounds the well would hopefully contain the Romneya's wandering roots. Also the sides of the well would provide some support for the stems of the poppy which can flop around quite a bit. The Romneya is also drought tolerant like the lobelia.
I planted 3 of these poppies in the well and they have done gratifyingly well. Sometimes this poppy is difficult to get established but that was not the case here. Last summer as I was looking at the floral display in this bed, I thought that some blue flowers would be nice there too. We have an area that used to be a compost heap by our garage, and is now becoming lawn. I had noticed that there were some seedlings of a blue flowered echium there, so I transplanted a few of those to this well and now they are flowering! So now I have a red (if you can call the lobelia flower red), white and blue planting scheme going.