Monday, 17 November 2008

Over-wintering Cannas

A rhizome is not a bulb. In botany, a rhizome is a horizontal stem of a plant that is usually found underground, and in Canna sending out roots and shoots from its nodes. Rhizomes may also be referred to as creeping rootstalks, or rootstocks. In general, rhizomes have short internodes; they send out roots from the bottom of the nodes and new upward-growing shoots from the top of the nodes.

For Cannas, the rhizome is used by gardeners to propagate the plants by a process known as vegetative reproduction.

So how does a rhizome differ from a bulb? A bulb is an underground vertical shoot that has modified leaves (or thickened leaf bases) that are used as food storage organs by a dormant plant. A Canna is never dormant, it is growing 52 weeks of the year. That is the bit that so many get wrong when handling their Cannas over the winter.

In the northern hemisphere, in England, our Cannas are growing outdoors for 6 months and spend the remaining 6 months being abused indoors! We see books and articles telling us to dry them out, cut of the roots and then leave them in boxes of peat until the spring. Watering is hardly ever mentioned. So all the Canna will be doing is draining its energy reserves stored in the rhizome to keep life growing. If it had a bad growing summer before it may not have enough food reserves to keep going and will just give up life. We have all seen rhizomes that do that.

So, what is the best way to over-winter a Canna. It certainly is not to leave it in a pot in a garage and ignore it for 6 months. While it may still have roots, it is in soil that the ever-hungry Canna will have depleted of minerals during the summer. Although it has roots, but no food, means it is drawing on energy in the rhizome during the winter months to keep going. The least that should be done in that case is to add fertilizer to the pot, lightly water and ensure that it never dries out. It will not produce any starch to plumpen up the rhizome when it has no foliage, but it will not deplete the rhizome of its stored energy.

There is an urban myth that says that if the first leaf in the spring is bad, then the Canna has virus. A little thought and understanding would reverse that, and the wisdom should be that a good first leaf indicates great health, but while early leaves may not be good, by the time the plant is growing with roots in well-treated soil if it is not producing better and better leaves, then it possibly has virus.

We often see what happens when you buy dry rhizomes from garden centres. They are usually withered and depleted of any stored energy. They are grown over the summer and people are disappointed. However, if given TLC during that first summer and over-wintered correctly then the plant will often amaze the next year, unless it has a bad infection of virus, which is a different matter altogether. So, I always recommend waiting until the second year before passing judgement.

So whatever method you use to store your Cannas over-winter, I would recommend that you ensure that it is kept ticking over for that long winter period.

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