From the
The Colorado Dahlia Society
http://www.dahlias.net/dahwebpg/PotTubers/PotTubers.htm
SOME NOTES ON POT ROOTS
by Walter & Kit Jack1
One may plant roots (clumps) directly, take cuttings, or cut up the pot roo=
ts the way one would any other field grown clump. The Jacks recommend repot=
ting pot tubers and starting them growing before deciding how to handle the=
m. Once the new growth is 2 to 3 inches high, one can divide the clump or s=
tart taking cuttings.
Suppose you plant an intact pot root, a divided pot root, and a rooted cutt=
ing, all at an equal stage, on the same day. Which would bloom first? In th=
eir experience, intact pot roots tend to flower 1 to 2 weeks earlier than t=
hose 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 t=
he root structure to get underway. However, from the time you start a pot r=
oot, 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 s=
harp 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 cutti=
ngs. This comparison probably varies by cultivar. For example, Camano Cloud=
has a reputation for producing better blooms from cuttings than from tuber=
s (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 o=
ne might compile a list of varieties that do best from each method of propa=
gation, 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 m=
uch 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 ro=
ots since they start without any mother root. This difference may explain w=
hy 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 cul=
tivars tend to produce better results from cuttings and others tend to do s=
o from tubers. However, in general, pot roots and tubers produce clumps tha=
t 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 outsi=
de under shade cloth to harden them off. They start their roots in 4 inch p=
lanter 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 root=
s, 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 thr=
eat of frost is past. Because they live in extreme Southern New Zealand, th=
eir growing season is short. They can have frost as late as mid November an=
d 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 fie=
ld. When they plant out, their pot roots usually have 6 to 10 inches of pla=
nt growth.
Growing Your Own Pot Roots
During the flowering season, Walter and Kit carefully mark 2 to 3 especiall=
y vigorous and healthy field tubers to propagate. They take the tubers from=
those plants, place them in polystyrene trays with a mixture of 60% pottin=
g mix and 40% coarse sawdust, place them on heat pads, and start them growi=
ng on August 1 (corresponding to February 1) with at first only a light wat=
ering. As growth gets underway, they step up the watering. They take two cr=
ops 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 fr=
om 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 f=
rom too much shock, especially if the weather is hot and sunny. Cuttings sh=
ould 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 d=
ry out but keep an eye on the cuttings and mist them lightly periodically t=
o keep the leaves healthy. The harsh treatment forces the rooted cuttings t=
o 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 rele=
ase Osmacote, plus a fungicide. The fungicide is most important because it =
prevents dampening off of the new cuttings. If your potting mix does not co=
ntain a fungicide, spray with Benlate or a similar fungicide immediately af=
ter repotting. If your potting mix does not contain time release fertilizer=
, use a balanced Osmacote or Once brand 3 to 4 month time release fertilize=
r at the rate the manufacturer recommends.
Once their potted cuttings have started growing in their greenhouse, they p=
lace them on sawdust pads about 4 to 5 inches apart. The close placement pr=
events the plants from growing too vigorously. One wants root growth, not t=
op growth. Some varieties (such as Cheyenne) make long, robust tubers that =
like to escape the planter bag. For these varieties, they lift the tuber ha=
lf 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 pr=
event 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 ou=
tside the bags and make them useless.3 A monthly spray with a combination f=
ungicide/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 t=
oo 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 fe=
d and watered plants can form small, soft, poor keeping tubers.
The Jacks apparently grow most of their pot roots under shade cloth. They g=
enerally 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 co=
st of clay pots for the high number of pot roots they grow would be prohibi=
tive. For home use, a dahlia enthusiast can use small containers such as fo=
r 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 th=
e 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 McClar=
en recommends letting the lifted pot roots dry out for a couple days and th=
en wrapping them in newspaper to store for the winter.)
If you find your pot roots tending to be soft or rotting, one problem may b=
e that the soft or poor keeping ones could be immature. Some varieties matu=
re 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 di=
fficult 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 b=
ack 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., Inve=
rcargill, Southland, in extreme southern New Zealand, are among the most su=
ccessful dahlia exhibitors in New Zealand. Belle Fleur Gardens is also one =
of the premier international commercial dahlia suppliers. The Jacks, origin=
ators of the Oreti line of dahlias (including the beautiful Oreti Kirsty, B=
B 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 so=
me hints from their extensive experience. Alan Fisher helped turn their let=
ters into an article and added some editorial comments.
2 The Jacks use 4x4 inch black polythene bags that are approximately 4 inch=
es deep. I am unaware of similar bags being sold in the U.S. The closest on=
e could probably find would be 4 inch square plastic pots. The bags are pre=
sumably much less expensive than pots for growers who plant thousands of ro=
oted 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 i=
s too large or the roots grow through the drainage holes and form outside t=
he pot.
4 The Jacks write,
A tunnel house is very common here, similar to a glass house but in the pla=
ce of glass. It has a pull-over polythene cover, probably referred to in th=
e 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 Dural=
ite (a fiberglass).
March 1995
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