Introduction
Annual hill strawberry plantings are
generally established by using either fresh dug or dormant coldstored “frigo”
plants. Both fresh dug and dormant plants are relatively inexpensive. However,
fresh dug plants are generally not commercially available
until October when they are dug by nurseries for large California and Florida plantings.
October is too late for planting in Northern Utah, where optimal planting dates
are in late August to early September. Dormant plants are generally dug in the late
fall or winter and cold stored for planting in the spring and early summer.
However, by September most of the plants have been in cold storage for such a
long period of time that plant viability is decreased (Hokanson et al., 2004). Plug
or tray plants (Figure 1) offer an alternative approach for obtaining
strawberry starts for use in annual hill and high tunnel production systems.
While plug plants are more expensive than fresh-dug or dormant plants, they
typically offer the appropriate level of initial vigor for fall planting.
However, commercial availability of plug plants is extremely limited. Currently
there are no commercial nurseries in the Intermountain West producing plugs and
shipping costs for fresh plants make ordering from distant nurseries
prohibitively expensive. Plug plants are relatively
easy to grow and can be produced in a small greenhouse or cold frame. Dormant cold-stored
mother plants are planted in the late spring/early summer to produce runner
tips for propagation. If mother plants are ordered in January or February,
there are generally a large number of varieties to choose from. Most suppliers
will store plants in their specialized storage facilities at no additional
costs until growers are ready to plant.
Producing the runner
tips
Site selection and establishment - The first step in producing strawberry
plug plants is to produce the runner tips. Runners are produced from mother
plants, which can be grown either in a greenhouse or field. Plants can be grown
in the soil or in peat-filled grow bags placed on the soil or on benches.
Runner production is favored by high temperatures and long days, hence outdoor
production would be limited to the summertime. With either system (outdoor or greenhouse)
care should be taken to prevent runners from coming in direct contact with
soil. In the greenhouse, suspended growing systems (Figures 2 & 3) are used
to prevent runners from coming in contact with soil. In the field, a
combination of straw mulch between rows and plastic mulch within the row will
prevent runner tips from contacting the soil. In the greenhouse, the first
runner tips can be expected about 8 to 10 weeks after establishing the mother
plant (Durner et al., 2002). The number of runners produced per plant will
increase over time and as the mother plants get bigger. However, the number of
runners per mother plant and the time from establishment to runner production
will vary somewhat by cultivar. In some circumstances it may be advantageous to
establish plants 3 to 4 months before runners are to be harvested in order to
obtain more runners per mother plant.
Greenhouses provide the ideal setting
for runner production. Greenhouses can be managed to provide ideal day time
temperatures (above 75°F) and long photoperiods (about 16 hours). With
supplemental heat and light, runner production can occur year round. Elevated
planting beds in the greenhouse facilitate runner harvesting. PVC rain gutters
filled with a soil-less potting mix provide a simple and efficient elevated production
system (Figure 2). Plants are spaced 9 to 12 inches apart in the gutters. The
resulting small root volume requires frequent irrigation and fertilizer application.
Drainage holes should be drilled in the bottom of the gutters to prevent water
saturation of the soil. Irrigation events will vary with different planting configurations;
however, the principles are the same. Plants should be watered often enough
that the soil doesn’t dry out and cause the plant to wilt. Each irrigation
cycle will normally last until water starts dripping out the drain holes, which
is an indication that the soil profile is full. During peak runner production plants
usually need to be irrigated once every 3 to 4 hours, with each irrigation
event lasting 3 to 4 minutes. This irrigation frequency can be easily achieved
with a simple time clock and automatic valve.
Tip harvest and
propagation
Runner tips should be harvested when
root initials (little white or brown pegs, Figure 4) are present on the runner tip.
Root initials should not be longer than ½ inch. Additionally, at least two
trifoliate leaves (first leaves that appear from the runner tip) are needed and
should be between 2½ and 4 inches in length (Durner et al., 2002) (Figure 5).
Runner tips where the oldest trifoliate leaf is larger or smaller will have
limited success in establishment. Depending on individual needs and desire for
uniformity of runner tips, tips are generally harvested every 10 to14 days.
Sorting tips by size will prevent larger plug plants from crowding out the
smaller ones in the tray (Durner et al., 2002; Takeda and Newell, 2006). Fifty
cell plug trays with approximately 7 cubic inches per cell work best for
strawberry plug production. Runners should be removed such that the trifoliate leaves
are not damaged and approximately ½ inch of the runner is left to be used for
an anchor when planting tips into plug trays (Figure 5) (Durner et al., 2002;
Takeda and Newell, 2006). Runner tips should be planted immediately after
harvest. In commercial operations, runners are usually planted or cooled to
32°F within 45 minutes. If runner tip storage is necessary, tips should be stored
at 32 to 34°F at 95 percent humidity for no more than 1 week (Durner et al.,
2002). Tips should be planted such that the root pegs and anchor are just below
the soil surface, with the leaves and as much of the developing crown as
possible remaining above the soil surface (Figure 6). The soil should then be
pressed lightly around the runner tips to hold the plant in place. After the
runner tips have been planted, they need to be protected from the wind, and
leaves need to remain moist until the plant can establish a new root system. Maintaining
moisture on the leaf is usually accomplished with a misting system. Misting the
plants intermittently for 7 to 12 days is generally sufficient. For the first 3
to 4 days, misting should be more frequent and for shorter periods of time
(every 5 minutes for about 10 seconds). After 3 to 4 days, the misting time may
be increased to 30 seconds every 12 minutes. As time goes by, the misting
interval should remain at about 12 minutes, and misting time should gradually
be decreased as the plant roots become more established. Protecting plants from
wind, heat, and direct sunlight will provide the best results. Shaded
greenhouses or shade houses with protection from wind provide the best environment.
After the misting regime is finished, plants should be allowed to harden off in
the greenhouse for 1 to 2 weeks prior to their establishment in the field or high
tunnel (Durner et al., 2002). Generally, a runner tip will produce a
well-rooted plug in about 4 weeks.
