Allium 'Millenium' is indeed a beautiful plant known for its striking spherical purple flowers and its ability to attract a variety of pollinators outdoors. However, transitioning this plant to an indoor environment does pose some challenges, since it's more conventional for outdoor gardens.
When it comes to indoor cultivation, light is a key factor. Alliums generally prefer full sun, so you’ll want to place it in a location that receives plenty of bright, direct sunlight. A south-facing window would be ideal. If natural light is limited, consider supplementing with a grow light to meet its needs.
For soil, a fast-draining potting mix is crucial to prevent the bulbs from becoming waterlogged. You can use a cactus or succulent mix or amend regular potting soil with sand or perlite to improve drainage.
Watering should be moderate. Allow the soil to dry out between waterings to mimic its natural conditions, as too much moisture can lead to bulb rot. Over winter, reduce watering further, as the plant will enter a dormant period.
Temperature-wise, Allium 'Millenium' should adapt well to typical indoor temperatures, though it thrives in cooler conditions during its dormancy. During this period, slightly cooler temperatures similar to outdoor fall conditions would mimic its natural cycle.
It's important to note that even though it can survive indoors, Allium 'Millenium' might not reach its full potential in terms of flowering when kept inside. If you notice it struggling, consider moving it outdoors during the growing season if possible, or providing the conditions it requires more precisely.
Lastly, the RHS and home gardening resources often emphasize the advantages of outdoor planting for alliums, notably due to their need for space and seasonal changes, which can be challenging to replicate indoors.
If you’re committed to trying it indoors, you might want to experiment a bit with its conditions to see what works best in your home. Good luck with your plant journey!