by Walter & Kit Jack1

One may plant roots (clumps) directly, take cuttings, or cut up the pot roots the way one would any other field grown clump. The Jacks recommend repotting pot tubers and starting them growing before deciding how to handle them. Once the new growth is 2 to 3 inches high, one can divide the clump or start taking cuttings.

Suppose you plant an intact pot root, a divided pot root, and a rooted cutting, all at an equal stage, on the same day. Which would bloom first? In their experience, intact pot roots tend to flower 1 to 2 weeks earlier than those that have been divided. Green plants from cuttings tend to flower 2 to 3 weeks later than the divided clumps, because it takes a bit longer for the root structure to get underway. However, from the time you start a pot root, you would need a few extra weeks for plants from cuttings, because you need to add the time required for the cutting to root. If you plant a pot root and it has more than 1 shoot, remove the surplus with the point of a sharp knife.

Which produces superior blooms, an intact pot root, a divided pot root, or a rooted cutting? In general, the Jacks find no difference in bloom quality among plants from undivided pot roots, divided pot roots, tubers, or cuttings. This comparison probably varies by cultivar. For example, Camano Cloud has a reputation for producing better blooms from cuttings than from tubers (if they keep) or pot roots. (Susan Ambrose once wrote that Camano Cloud frequently does not put out feeder roots when planted from tubers.) While one might compile a list of varieties that do best from each method of propagation, we do not have such a list. If a cultivar produces huge tubers and you plant the entire tuber, there is a danger that the plant may rely too much for nourishment on the mother root and not put out enough feeder roots to have maximum vigor. Rooted cuttings, in contrast, must put out feeder roots since they start without any mother root. This difference may explain why rooted plants sometimes produce superior blooms compared to plants from tubers it may also explain why the smaller pot roots often produce superior plants and blooms to unrestricted field tubers. One guess is that some cultivars tend to produce better results from cuttings and others tend to do so from tubers. However, in general, pot roots and tubers produce clumps that are easier to handle and divide compared to clumps from rooted cuttings.

The Jacks start their pot tubers in tunnel houses and then shift them outside under shade cloth to harden them off. They start their roots in 4 inch planter bags and keep them until they average 4 to 5 inches of growth above the planter bags.2 Around mid November (which corresponds to mid May in the Northern Hemisphere), they break away the potting mix, spread the new roots, and plant the started pot tubers about 4 inches deep.

The Jacks start the pot roots before planting them out for several reasons. By determining that they are healthy, they have no crop failures and thus avoid gaps in their display beds. They delay planting in beds until all threat of frost is past. Because they live in extreme Southern New Zealand, their growing season is short. They can have frost as late as mid November and as early as the first part of March. By starting pot roots, they can also determine any growth problems (such as virus) before they plant in the field. When they plant out, their pot roots usually have 6 to 10 inches of plant growth.

Growing Your Own Pot Roots

During the flowering season, Walter and Kit carefully mark 2 to 3 especially vigorous and healthy field tubers to propagate. They take the tubers from those plants, place them in polystyrene trays with a mixture of 60% potting mix and 40% coarse sawdust, place them on heat pads, and start them growing on August 1 (corresponding to February 1) with at first only a light watering. As growth gets underway, they step up the watering. They take two crops of cuttings from each set of tubers, although they can take more it the shoots remain vigorous. They find they do not obtain worthwhile results from weak growth. As they finish with tubers, they remove the used tubers and replace with new tubers to start for cuttings.

They cover new trays of cuttings for a day with newspaper to protect them from too much shock, especially if the weather is hot and sunny. Cuttings should receive bottom heat for up to 2 weeks.

The Jacks pot the cuttings starting 3 to 4 weeks after they take them. They recommend leaving the trays of rooted cuttings up to a week without water (depending on growing conditions) before potting them. Let the mix almost dry out but keep an eye on the cuttings and mist them lightly periodically to keep the leaves healthy. The harsh treatment forces the rooted cuttings to put out lots of new growth as the plant looks for moisture. By drying out the rooting medium, you keep root damage to a minimum, because dry potting mix breaks away from the young, tender roots more readily than does heavy, wet potting mix.

The potting mix they use contains peat, river sand, bark, 6 month time release Osmacote, plus a fungicide. The fungicide is most important because it prevents dampening off of the new cuttings. If your potting mix does not contain a fungicide, spray with Benlate or a similar fungicide immediately after repotting. If your potting mix does not contain time release fertilizer, use a balanced Osmacote or Once brand 3 to 4 month time release fertilizer at the rate the manufacturer recommends.

Once their potted cuttings have started growing in their greenhouse, they place them on sawdust pads about 4 to 5 inches apart. The close placement prevents the plants from growing too vigorously. One wants root growth, not top growth. Some varieties (such as Cheyenne) make long, robust tubers that like to escape the planter bag. For these varieties, they lift the tuber half way through the growing season and clip the roots.

Cover the pots with 1 to 2 inches of sawdust to keep the moisture in and prevent the potting mixture from going hard on the top and shrinking from the sides of the bag. If you plant the bags any deeper, the roots will form outside the bags and make them useless.3 A monthly spray with a combination fungicide/insecticide and an occasional clipping to prevent the plants from flowering (except one or two blooms to verify the variety) constitutes the extent of the care they receive.

The Jacks recommend growing the pot tubers hard -- no foliar feed and not too much water. If the climate permits, grow them a little on the dry side -- the tubers will be firmer and will keep better during the winter. Over fed and watered plants can form small, soft, poor keeping tubers.

The Jacks apparently grow most of their pot roots under shade cloth. They generally grow the more difficult to tuber varieties (such as Fern Irene and Margaret Ann) indoors, in their "tunnel houses."4

Clay pots would be preferable to the planter bags the Jacks use, but the cost of clay pots for the high number of pot roots they grow would be prohibitive. For home use, a dahlia enthusiast can use small containers such as for margarine or dairy products. Keep the drainage holes small to prevent the roots from escaping. To winter the pots tubers, cut off the tops, leave the tubers in their containers in the growing medium, and then repot them to start when you are ready to use them for the following season. (Bill McClaren recommends letting the lifted pot roots dry out for a couple days and then wrapping them in newspaper to store for the winter.)

If you find your pot roots tending to be soft or rotting, one problem may be that the soft or poor keeping ones could be immature. Some varieties mature late. Some varieties always make good pot tubers while others are very "shy." Most healthy, vigorous dahlias make good pot tubers. Any dahlia that has health concerns or is not a vigorous grower, normally makes small or difficult to keep pot tubers. When an old favorite starts letting them down, the Jacks retire it to the "paddock block" (Kiwi term for a field) for 2 to 3 years without any fertilizer, restrict it to one set of blooms, and then cut it back. This treatment revives many varieties and makes them bounce back better than ever. Those that improve they try again. Those that fail to respond they discard.

1 Editor's Note: Walter & Kit Jack of Belle Fleur Gardens, No. 4 R.D., Invercargill, Southland, in extreme southern New Zealand, are among the most successful dahlia exhibitors in New Zealand. Belle Fleur Gardens is also one of the premier international commercial dahlia suppliers. The Jacks, originators of the Oreti line of dahlias (including the beautiful Oreti Kirsty, BB C DB, and highly successful Oreti Duke, Porn Pr), plant and sell only pot grown tubers. In response to a request from the ADS Bulletin, they sent some hints from their extensive experience. Alan Fisher helped turn their letters into an article and added some editorial comments.

2 The Jacks use 4x4 inch black polythene bags that are approximately 4 inches deep. I am unaware of similar bags being sold in the U.S. The closest one could probably find would be 4 inch square plastic pots. The bags are presumably much less expensive than pots for growers who plant thousands of rooted cuttings each year.

3 When the stem has roots between it and the formed tuber, it is difficult to get the pot root to keep. This condition arises sometimes when the pot is too large or the roots grow through the drainage holes and form outside the pot.

4 The Jacks write,

A tunnel house is very common here, similar to a glass house but in the place of glass. It has a pull-over polythene cover, probably referred to in the U.S. under another name. One we use is 100 x 26 feet; another is 50 x 26 feet. For propagating, we use a 50 x 30 it. hothouse constructed from Duralite (a fiberglass).

March 1995


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