THE GROWING WORLD OF DAHLIAS

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POT TUBERS, THE WAY TO SURVIVE

by Dave Bates

What is a pot tuber? Basically it is the tuber produced from a plant grown in a restricted root environment such as a pot, rather than being planted in the open ground. It is significantly smaller than its unrestricted relative. Their origins are based on several parameters, mainly economic rather than horticultural. With escalating costs of postage and carriage, nurserymen are not able to send field grown (unrestricted) tubers to their customers. Imagine the weight of half a dozen good sized tubers and consider the cost of posting them when a Christmas card costs over 20p these days. The customers would not be willing to pay the cost of carriage, and hence orders would reduce. A further problem is the space taken to grow unrestricted tubers, and the consequent reduction in number that the nurseryman could supply. Reduced numbers involves extra cost per tuber to allow a break even, and again this is not really acceptable if the business is to flourish. The Dutch plastic bag dahlias are all restricted tubers. They have to be horticulturally sound in order to encourage customers to buy more the following year. Thus the trade is well into pot tubers, and naturally dahlia enthusiasts have seen the advantages themselves.

The soil where I grow my plants is quite fine and light, over a clay and pebble sub-soil, so water drains quickly through and then runs off on the sloping subsoil. Plants do not make good tubers, many producing fine whip like roots that are subject to drying out too much during the winter. This is especially the case with the thick stemmed varieties such as the Cupid family, and these often fail with stem rot as well. I suffered many heart breaking Winters and expensive Springs until I was introduced to pot tubers!

Since then, stock losses have been considerably less. Since the plants are restricted, the stems are much thinner than those grown normally. There is less opportunity for them to rot, and as the tubers are left in the pots until propagating time, the soil in which they have grown prevents them from drying out too much. After that, I found many other benefits. I have an 8' by 6' greenhouse and drying space for field tubers in October/November is limited such that it takes a few weeks to get all the tubers lifted and dried. During these weeks, those left in the ground start to deteriorate, and later liftings are the most vulnerable to loss. Pot tubers take far less space and can be left on their sides under the benching. There is no need for air circulation round them and it is possible to lift the entire stock of tubers and move them to the greenhouse in one weekend.

No matter how large the greenhouse, propagating space is at a premium. By judicious trimming, it is possible to fit two or three field tubers to a standard seed tray, whereas about a dozen pot tubers will fit in one. If a field tuber fails, a large amount of space has been wasted, but only a small amount with the pot tuber. Not only that, but the pot tuber fires into life much more rapidly than the bigger ones, so there is more chance to replace the bad ones on the bench and hence recover the situation. The plants used for pot tubers are usually the early ones where the stem has hardened prior to planting, as these never do very well if grown for large plants. They can be retained in 3.5inch pots or moved to 4 or 5inch pots. I have found that potting in soil-less compost is not very good for tuber production and keeping, and I prefer compost which is more open, usually produced by mixing the peat base with John Innes No 3
and some grit and sand.

At planting out time, the pots are buried to their rims, they can be pot thick if space is limited. During the growing season, they must be sprayed on a regular basis to counteract pests, and foliar fed. Towards the end of the growing season, a high potash feed should be used to ensure the ripening of tubers. It is important to watch the health of these restricted plants, any showing the slightest sign of virus should be rogued and burned before there is any danger of it spreading. When the plants flower in the crown bud, (there is no need for stopping), ensure the variety is as labelled as there is no point in propagating from a wrongly labelled tuber. Around the first weekend of October, the stems are cut down to about 2", even if they are still green and the pots dug out and moved to the greenhouse. The major disadvantage of this technique is that there is little opportunity for stock selection with these plants, but they should have originated from the best stock of field tubers in any case. I try to propagate from a mixture of pot and selected field tubers to overcome this difficulty, but there are always some varieties whose field tubers refuse to keep. As you recognise these types of varieties it is important to sacrifice some potential exhibition plants for the following year's stock.

This article written May 1992

Last updated February 02, 2005

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