Alliums flower in that surprisingly "bald patch" in May when everything else is about to burst into flower but has not quite made it yet. That makes them valuable anyway, but they are also have some of the most strikingly architectural flowers you can grow in the UK. Add to that the fact that they are easy and it is hard to understand why they are not more widely used. It may be because people believe that as members of the onion family p[eople think they smell of onions. Well, we have half a dozen different varieties from our range of alliums for sale in our relatively small garden at home and they have multiplied over the years, so we can have better than 100 in flower at once. If you get close there is a faint oniony smell, but most are tall and set back inthe borders and so it is simply not noticeable. Even by a wife who has a nose like a bloodhound! Having said which, the scent is enough to help deter pests such as aphids which depend far more on smell than we do.
They are all pretty good. But it might help if you look at the choices like this:
The three above will be fine at the back of a wide border where you have taller subjects. Otherwise for height use Red Mohican (red) or and Allium Silver Spring (mainly white with a little purple). At the front of the border the shorter but huge flowered metallic blue Allium Christophii looks fantastic.
The standout performer in grassy areas is Allium hollandicum Afflatuense which will also stand a little shade. In woodland, especially where the soil is moist, then Sicilian Honey Garlic is the allium of choice.
Autumn & early winter - which is peak bulb planting time for so many bulbs - is also the best time for alliums. The ideal months are from September to mid November but as late as the middle of December is acceptable. Bulbs are less sensitive to temperature extremes than most plants (they are pretty well insulated under ground), but it is best not to plant whne temperatures are
With the notable exception of Sicilian Honey Garlic (which likes damp soil in shade), the other alliums as members of the onion family love the sun. This is really important for them to flower well. They also like relatively deep rich soil. Not too big an ask as the sort you find in rose beds, herbaceous borders and kitchen gardens is ideal. And the annual mulch they get in those places also suits them very well. You can lift alliums every year, but to be honest they do better if they are allowed to settle in and I prefer to only lift clumps when they are over. On that basis, plant them where you do not have to disturb them for 3-4 years.
For bulbs such as tulips and daffodils, a warm and sunny location with good drainage is best. These are bulbs which originate from mainly dry summer climates so an attempt to imitate this will work in their favour.
With alliums it is a good idea to plant soon after receipt. They are big bulbs and lose moisture relatively quickly when out of the ground. So prepare their planting site early. As already mentioned, a sunny spot with ferile soil and good drainage is best. unless your soil is already very good, adding well rotted garden compost is one of the best investments you can make for a fabulous display in May and June.
Some alliums are very big bulbs indeed and are best planted in individual holes. Planting depth is important; thise big flower heads offer a lot of windage and the bulbs need to be well anchored. So try to make sure that they are covered with at least three times the height of the bulb in soil. Remember that the top of the allium is pointed, but if in dount plant the bulb on its side. Fill in the holes, tread down well and mark clearly
No reason why not. Just make sure the pot is deep and fairly large and the compost is kept moist in the growing season. The other thing to think about is underplanting. Alliums are not great grown all by themsleves as their leaves are fading and look relatively unattractive just as the flowers open. Massed late flowering tulips can look good. Tey Spring Green or White Triumphator.
Aftercare is a piece of cake. Remove the leaves when they have withered. Leave the allium flower heads as they set seed when they look as striking as they did in bloom or cut them for dried flower arrangements. Mulch in early spring along with your roses or the nerbaceous border. If flowering dimishes, it will be because they are overcrowded in which case mark the clump in June (so you can find it later) and then lift, divide and replant in September.