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Romneya - Growing Guide
Growing Romneya coulteri
Commonly known as ‘Tree Poppy’
Californian tree poppies are semi-herbaceous shrubs widely admired for their huge white flowers and their prominent yellow stamens which begin flowering in early July and then, progressively, on through to autumn. These plants are however not without difficulties and concerns for gardeners and these need to be understood.
Romneya can be very invasive plants. Although not in the category of giant knotweed for undermining roadways and foundations they are probably best not grown against the house. Their long underground new growth tendrils will spread quickly through a bed and need to be chopped back annually to stop them undermining nearby slabs or patios.
R. coulteri will grow 4-5ft multiple shoots each year when established. In a very mild winter these may never fully lose their glaucous greyish leaves entirely. The question is then whether to cut them to the ground or not as you would anyway as a matter of course in colder areas. Romneya are fully hardy so late frosts are no risk. We generally prune our romneya back to the ground every two or three years. We find that the vigorous new shoots are more floriferous and more manageable as a result.
Romneya growth shoots can blow over readily in the sunny positions which they like. Although fully wind hardy too, you may want to consider using some plant supports later in the flowering season.
Then gardeners puzzle as to how to propagate this attractive plant as they simply will not root from cuttings. You can try chopping off the side suckers but you will find that they often have little root attached. The way to propagate these plants is to create a sandy bed near your plant and cover it with a slate slab or stones. Remove the covering in the winter and chop through the roots into 6-9in sections. The stems and roots will quickly regrow as individual small plants which can be lifted and moved elsewhere.
When planting romneya prepare the bed well as they hate being moved. Plenty of dung and leaf mould should be used in the planting area and give them plenty of space to spread into. A winter mulch after pruning each year will improve the growth rates next year of this outstanding garden plant.