It will come as no surprise to those who grow vegetables, that the gorgeous ornamental Allium is a direct cousin to the ordinary onion that we grow for show or for eating.
For those who have never grown onions - the vegetables - it always seems to amaze, that the colourful - even spectacular - blooms of the ornamental Alliums can simply be 'onions'!
The pictures below will show the similarities.
On the left we have the vegetable onion - Allium cepa which has been left to grow to seed (not a good idea if they are for the plate!) On the right, the ornamental Onion - Allium giganteum. There are several other types of ornamental Onions - or Alliums, being ornamental Garlic and ornamental Chives.
Alliums are normally late spring flowering bulbs, but also flower into the summer, and hold their seed heads well into winter. There are also Alliums that flower in the Autumn.
Some Alliums are bulbous, whilst others have the thick roots normally associated with rhizomes. For the purpose of this page, they are all herbaceous perennials, but are almost unique amongst bulbs and even other types of plants, in that the foliage of most - certainly the larger types - will have withered by the time the flower heads form.
As with the edible Onion family, there are a number of types of Alliums with slightly differing flowering times, different habits of growth, and an assortment of flower colours.
Basically, there are ornamental onions, garlic and chives. Some are edible, some form bulbs, others form fleshy roots or rhizomes. All are perennials, in that they re-grow year after year. Some are frost hardy, whilst others are tender and will need winter protection by lifting and drying off - ready for storing in the winter months.
There is a narrow rage of flower colours normally centred around reds, blue and purples. However, there are also white form and a few yellow Alliums to be had.
The hardy types that we grow in our gardens, are happy in a wide range of soils - sunny positions preferred. However, most will also grow in light or dappled shade. Moist fertile soils are the favoured - as with most plants - but adaptable Alliums that they are, they have learned to cope with dry conditions as well - though benefitting from regular watering in the growing season up until flowering time flowering. The wild Garlic - Allium ursinum - is happiest in woodland conditions - as damp as you like!
The fact that the foliage dies off by the time the flower heads mature can make for added interest if planted in large drifts in otherwise bare soils or low growing plants. Upright stems with large globes of flower atop can be quite spectacular. There is also a good case for under-planting with low growing plants such as forget-me-nots (Myosotis). Pansies and smaller flowered violas are another choice.
Allium cristophii is one of the largest of all Allium flowers - seen here in June - but in spite of its size, the minimal foliage can be seen in background. Flowers of this Onion can reach 8in (20cm) across.
Alliums are good subjects for mixed containers, as their foliage is not too intrusive, and in any event dies down before most other plants need the space. If grown in containers, then the bulbs are best lifted for storage each autumn - or the container given added protection against hard frosts and freezing winter conditions.
When do you plant allium bulbs? A question mainly answered by whenever you see them for sale at the garden centre. They will be pre-packed, loose bulbs or container grown (expensive way of buying - but irresistible.)
When to plant allium bulbs is normally governed by your choice of either bulb or pot grown plant. The summer flowering Alliums are normally available in the previous autumn, and can be planted right away. Garden Center bulbs are often sold from autumn to spring, but are normally the first of the bulbs to disappear from the shelves, so keep a look out for them - or befriend your garden centre owner to find out when they will be displayed for sale to avoid disappointment.
Allium Bulbs should be planted at around twice the depth of the bulb from nose to base. So, the larger bulbs need to be planted with around 4 - 6in soil over the top of the nose. Smaller bulbs with a couple of inches of soil over top, The earlier you can plant them, the sooner they will settle in - ready for the oncoming winter.
Fleshy-root or rhizome-rooted varieties are best planted in the spring, with the emerging new growths at soil level - or even slightly higher. Do not bury completely as with the bulbs.
All types are best planted with compost in the planting hole - and a small handful of bonemeal. Otherwise use Osmocote pellets, which will give the plant all the food it needs up to and after flowering time. Alternatively, liquid feed weekly as soon as the foliage starts on bulb types, and right after planting with the rhizome types of Allium.
As with their edible onion counterparts, ornamental onions - Alliums - have a few family related pests and problems.
Onion White Rot - very descriptive - attacks the foliage and bulb. NOT a common disease with Ornamental Onions. First above ground signs are a yellowing of the foliage. However, bear in mind that the ornamental onion's foliage turns yellow naturally and dies off before or during flowering! If the flower and stem is in good health, then there is no need to suspect onion white rot. If white rot is a problem, then there will be tell-tale signs at the base of the bulb if and when you lift them. A good helping of compost at planting time seems to alleviate the problem. There is no control - chemical or otherwise for this disease. Just don't plant any more alliums in the same place - for years!
Downy Mildew - Not a common problem, but if present, shows on the foliage or stems as rounded mares with a look of 'mould'! Any garden spray that is suitable for Botrytis will go a long way to alleviate this problem - if encountered.
Onion Fly - Does not normally attack border grown spring or summer flowering Alliums.