For years, we struggled to grow the Mediterranean/Balkan native spurge, Euphorbia characias…until we discovered its secrets. First, it isn’t a long lived plant to begin with…in most cases 3-5 years is it, so you’ll need to plant it where it’s likely to reseed. That would be well-drained slopes that are either mulched or covered with gravel.
Secondly, after it flowers in spring with its stunning show of yellow flowers, remove most of the flower stalks as soon as flowering has finished, except those needed to produce new seedlings (the flowers are also great to use in floral arrangements). If not, the seed stalks use up energy causing the plant to decline much faster. We’ve now allowed this to seed throughout the slopes in front of our house, and here is the result…a smattering of 3′ tall x 3′ wide clumps, photo taken mid-winter.
Although this section of the garden, planted in compost-amended sandy loam is irrigated, we typically don’t recommend irrigation for this spurge without excellent drainage. You’ll also read on-line that Euphorbia characias doesn’t like hot, humid summers…another example of fake gardening news that just keeps getting repeated without any concern for the facts.
We’ve also found Euphorbia characias to grow well in part sun under large trees, which keeps the soil dry. The plants will never be as dense as they are in full sun, but they survive and flowers. There is really not anything else that gives you this evergreen blue color and form in the winter garden.