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Suggestions from Kevin Larkin
August 19 2007
First of all, Kevin congratulates us (thanks Ray and Dave) on doing a better job growing
dahlias than many people he has seen who have been growing for 20 years. He admired
the watering system, the health of the plants, and the labeling. However, after attending
the San Francisco Dahlia Society show and noting some flowers of the same cultivar
twice the size of ours, I think the following suggestions are excellent ones:
Pests and Diseases
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Our plants are infested with Western Flower Thrips, evidenced by contorted
leaves and “shiny” spots on their undersides. This can be controlled using Entrust,
an organic spinosid of which Paul should be aware. Kevin recommends spraying
as needed to assure control. In addition, deadheaded flowers should be removed
entirely from the area so that thrip eggs inside the flower heads cannot hatch and
re-infest an area.
On some plants, powdery mildew is becoming a problem. This, combined with
thrip damage, is to blame for the lower leaf death we’ve been seeing in our plants.
This can be controlled by sulfur formulations; Kevin suggested This, a flowable
sulfur that is fairly common in use to prevent powdery mildew. In addition, he
mentioned that overhead watering, when applied very early in the day, helps
prevent powdery mildew by causing the fungal spores to lyse prematurely.
Mites were noticed on some plants, especially those near Greenhouse 8. Kevin
recommends using the beneficials used inside the greenhouses outside to control
this problem, and considers this not to be a serious issue at the moment.
Fertilization
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Kevin commended our early use of Peters 15-5-15 CalMag and said that he uses
the same early in the season. However, for best results, he recommended
switching to straight calcium nitrate at a continuous application rate of 200 ppm,
or 10 pounds spread over our population every three weeks. Kevin noted that he
feeds his plants very heavily, and they respond by making growth about twice the
height of ours, and flowers twice the size of ours; I can corroborate this based on
the sizes of some of the flowers I saw at the show.
General Plant Maintenance
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Some cultural practices will ensure larger, healthier plants and blooms. At
Corralitos Gardens, Kevin shears the population after the first flush of flowers.
This causes growths to elongate from closer to the main stem of the plant,
assuring a more robust plant less likely to topple over or break in strong wind.
In addition, a regular deadheading schedule should be created and maintained,
remembering to fully dispose of spent flowers to control thrip damage.
Deadheading cuts should be made 12-18 inches below the spent flower to assure
more sturdy growths and a more robust plant.
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A second series of wire should be installed next season, possibly with lateral
wires to further restrict the plants.
Plant could have leaders staked individually in order to further prevent a
sprawling or prostrate growth habit; staking and wiring should be done early in
the season.
To be sure that flowers are as large as possible, floral stems should be disbudded,
leaving only the primary bud (the one that grows in the center of the three-parted
floral stem).
Remove these
Leave this
Propagation Guidelines: Cuttings
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Based on the calendar he normally uses, Kevin said that for us to have blooming
plants for the first weeks in February, we need to take cuttings now.
Stick cuttings in Oasis;
put under mist with light
Sept. 1 2007
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Pot rooted cuttings in 4” pots;
move to 8” if bedded in pot
First early varieties flower
Oct. 1 2007
Feb. 1 2007
Cuttings should be taken from plants in the upper area, if possible, because they
appear more vegetative rather than producing flowers as do the plants in the lower
area.
Cutting should be taken of softer tissue if possible; tissue not in the sun, and
which is still the default stem color, is the preferred material to use. Hollow
stems do not root well.
If necessary, plants can be sheared back, or even cut to the ground to encourage
longer vegetative shoots; rooting percentage is typically 70-80%.
Cutting should be 4-5 nodes in length; the bottom pair(s) of leaves should be
removed before sticking the cuttings, and the flower bud (which should be
small) should be pinched out of the cutting. Additional pinches for developing
flower buds may be necessary to assure good vegetative growth prior to flowering.
Cuttings should be grown in 16 hours of light per day by extending the day length
with supplemental metal halide lighting, at the same temperatures as the plants
currently experience.
Cuttings should be grown under mist.
None of the plants on the “To Propagate” list (see database_spreadsheet.xls) are
considered difficult. Kevin recommended ‘Marilyn M’ and ‘Cheerleader’ as fast
flowering varieties, and said he would look at the spreadsheets and give
further information.
It is preferable to propagate by cutting and not lift tubers each year. Next year
we should plant everything in the ground propagate from cuttings alone.
Propagation Guidelines: Tubers
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Propagation from tubers should begin with cutting the stem back to 2-3 inches
above the soil line.
All plants in pots need to be pulled up and overwintered as tubers. They may be
planted in the ground next year.
Pots and any associated tubers should be lifted carefully; Kevin said some tubers
will have grown outside the pots, and great care should be taken to remove them
without breaking the neck. If the necks are broken the tubers will not sprout.
Roots should be gently cleaned of dirt. Kevin uses low-pressure compressed air;
in the past he used water, but feels as though the additional wetness contributed to
fungal problems over the winter.
Tubers should be carefully removed from the stem, keeping the neck and head
from breaking, and should be stored in zip-lock bags with vermiculite, one
cultivar per bag at 40º-60º F. for the winter.
Tubers should be potted up in early March into 6”-8” pots; the eye should be
visible at the soil surface. They should not be watered until they sprout.
Tubers will produce multiple sprouts, which should be thinned to the single
largest sprout prior to planting out. Planting should be done when sprouts are 3”4” in height.
General and Miscellaneous Additional Notes
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Dahlias planted outdoors should be cut to within 2”-3” of the soil surface when
cold temperatures make the plants appear unsightly. These plants will produce
multiple sprouts, which should be thinned to the single largest lead to encourage a
strong leader, which should be staked early to assure robust growth.
The label for ‘Dixie’s Winedot’ in the upper area is misapplied, and needs to be
moved to the plant in the area near greenhouse 8.
What we have labeled as ‘Bert Pitt’ is actually ‘Baron Chris’, which needs to be
changed in all the applicable places on the website and on the spreadsheets (I can
do this next year).
Kevin suggested as additional experiments some issues that dahlia growers
regularly find to be problematic:
 Why do some cultivars (and some growing conditions) produce leaves in
alternate or whorled arrangements rather than opposite, as dahlia species
genetics would seem to dictate?
 Why do some cultivars (and some growing conditions) produce extrafloral phyllary bracts lower on the stem, which negatively impact the
perceived “quality of bloom” at dahlia shows?
Notes as recorded by Tim Culbertson
cc: Virginia Walbot, Raymond von Itter, Dan Wilson, Kevin Larkin