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Always remember that when you buy containerised plants from your local nursery or garden centre that the corms will not always planted at the correct depth in the pot.
Corms may only be planted 1-2 inches from the surface of the compost, and thus if pots are planted at compost level to soil level the corms could be killed by harsh frosts in the winter. Also on planting this way the corms will be more drought prone in the summer and this will have a negative effect on flowering.
Always try to plant Crocosmia corms at a depth of 4 -5 inches, this lessens the risk of frost damage and ensures corms are deep enough to find water during the summer. Being deeper also means that the corms are cooler in the summer and will be less prone to drying out which can hinder their development.
I am often asked why Crocosmia give up flowering after a few years and only produce leaf. The simple reason for this is that the corms are too shallow and too close together; being too shallow and too close there is too much competition for water and nutrients and thus plants will not be able to produce flowering size corms. This is why Crocosmia should be split and divided every 3 or four years and re-planted at the correct depth. Ideally the ground around them being manured to ensure replacement of nutrients from which they will benefit. If the soil dries out quickly in the summer, or on sandy soil then mulching is required to keep corms nice and cool and help preserve moisture in the soil. Mulches can be of grit, woodchip or grass clippings, but my personal favorite is old well rotted stable manure where shavings have been used and not straw, and this also provides nutrients which will continually washed down to the corms, it's rather like using a slow release fertiliser.
Except where grown in long deep pots e.g. Clematis pots, containerised plants MUST be planted about 2 inches deeper in the ground than the level of the compost. In South Africa where Crocosmia come from, bulbs are often found deeper in the ground than you would think, and not just because of frost, but they've evolved this way, and to escape predation by mole rats and baboons, and trampling by grazing animals.
Deep planting can also be applied to other commonly grown African Bulbs in the UK and especially for Watsonias, which can be planted at 12 inches, and some Crinum
lilies, which can be planted at depths of over two feet. However, the planting depth of larger bulbed species is relative to bulb size and the larger the bulb the deeper it should and can be planted.
Where hardiness may be a problem, a colonial method of planting corms is probably the best method e.g. if corms are planted together in a group, a dozen or more then the group will be hardier and more likely to survive than if a corm is planted on its own, and because plants support themselves and provide shelter for each other, amongst other reasons.
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