Crown imperials are strong-growing bulbous plants, making rapid growth in the spring, flowering during April/early May and dying down in early summer.
They need a deep, rich, well-drained loam soil, preferably alkaline, and a warm, sunny position where they can be left undisturbed for many years.
Where soil conditions are less than ideal it is advisable to thoroughly prepare the site before planting, digging in some well-rotted manure or rich leafmould and grit. Simply putting grit in the bottom of planting holes on heavy soils will achieve little. Add a dressing of Vitax Q4 Fertiliser at about 140g per sq m (4oz per sq yd) before planting in the early autumn.
Plant bulbs deeply at a depth of at least 30cms (1ft) and a similar distance apart. Shallow planting will lead to poor flowering after the first year. If the soil is too wet to allow planting at this depth then it is not a suitable site. Consider constructing a suitable raised bed.
It is claimed that planting the bulb on its side may prevent rot when the bulb is dormant during the summer. Over a season, however, bulbs tend to right themselves.
Add a mulch of well decayed compost as the shoots emerge in the spring, after applying a general purpose fertiliser at the rates quoted above. Additional liquid feeding of high potassium (tomato) fertiliser during the growing season at fortnightly intervals is beneficial but start once bulbs have made a few inches of growth. Waiting until bulbs have flowered, as often recommended, is just too late as the new bulb for next year will have already formed.
Crown imperials rarely do well on heavy clay soils and, in such situations, are best grown in raised beds with good drainage or in containers. For the latter, use a mix of equal quantities of John Innes No.3 and multipurpose growing media with the addition of about 20 percent grit. Incorporate Vitax Q4 at 5g/litre. Do not use controlled release fertilisers which are unsuitable for spring flowering bulbs in containers due to excessive nutrient release.
In good growing conditions crown imperials will readily form large clumps. If a well-established colony begins to flower poorly then lifting in early autumn when dormant, thoroughly improving the soil and replanting, or moving to a new site may be sufficient to restore satisfactory flowering. The bulbs may take a year or two to re-establish.