December 2020 - 8

Cornell's tips for grafting tomatoes successfully
By Charles Gagne
Cornell University
Grafting can benefit your production
in many ways, such as improving yields
and increasing disease resistance. Follow
these guidelines, which have been
achieved through years of trial and error,
and see your profits grow.
Grafting is an age-old process of
connecting the vascular tissues of two
distinct plants. The aim of grafting is to
combine and utilize beneficial traits of
both the rootstock plant and the scion
plant (the top plant that yields fruit).
While the grafting of woody plants and
vines such as apples and grapes has been
common for years, grafting of herbaceous
plants such as tomatoes and melons has
only recently become popular. Today,
grafting of herbaceous vegetable plants is
widely used in intensive greenhouse and
tunnel production systems.
In the United States, tomatoes are
the most common crop grown under
protection in greenhouses and high
tunnels and their production in these
systems has benefited enormously
from grafting. The grafting of tomato
rootstocks - which are chosen for their
disease resistance, stress tolerance or root
system characteristics such as vigorous
vegetative growth - with tomato scions
- which are chosen for their desirable
fruit characteristics - has many benefits
to tomato growers. Grafted tomatoes
typically have higher yields, produce for
a longer time, have increased disease
resistance and may require less pesticides.
For large commercial growers,
rootstock and scion seed will be
selected and sent to a propagation
company who will grow the seedlings,
complete the grafting process, and then
send the grafted plants to the grower.
Although this is an expensive process,
it is ultimately much cheaper than the
time, labor, space and equipment it
Figure 1. A tray of rootstock, left, and scion,
right, seedlings before the grafting process
has begun. Photos: Cornell University
would take to graft the plants needed for
their operation. However, for smaller
operations, home growers or research
facilities, it can be useful to graft your
own plants to save money and allow for
more control (such as limiting potential
pests, diseases or viruses that could enter
from an outside facility).
If a grower decides to graft their
own tomato plants, it is first critical
that they have a good understanding of
the grafting process. It takes time and
practice to become skilled at grafting.
The following guide details the tomato
grafting process developed over years
of trial and error from researchers
and students in Cornell University's
Controlled Environment Agriculture
group. Included are also a few helpful tips
that the Cornell team has learned along
the way.
To begin, the grafting process can be
split into three distinct phases:
1. Pre-grafting/seedling growth.
2. Grafting.
3. Post-grafting/healing.
Phase 1: Pre-graft/seedling
growth
The first step in the pre-graft/seedling
growth phase is choosing the rootstock
and scion cultivars. The most common
rootstock cultivar used in commercial
tomato production is Maxifort. While
some growers may choose to use one
rootstock cultivar for large fruit scions
and another for small fruit scions, in
commercial practice Maxifort, or one of
its many genetic successors, is used with
practically all scion cultivars. Maxifort
is so widely used because it has vigorous
vegetative growth and has resistance to
many common diseases. However, other
rootstock varieties such as Estamino
or Beaufort are also popular among
growers. For scion cultivars, Merlice is a
common tomato-on-vine (TOV) variety,
as it has consistent fruit and growth, high
yields and resistance to powdery mildew.
Sweetelle, a popular and flavorful grape
tomato variety, is another commonly
used scion.
Once the cultivars are selected, the
seeds should be sown into a 98-cube
rockwool sheet (alternatively seeds can
be sown in other soilless media cells) that
has first been soaked with tap water for
10 minutes. The rockwool sheet should
then be placed in a growth chamber
or a different climate-controlled room.
Temperature in the chamber should be
maintained at 73° F/68° F (day/night)
and light should be provided at 260
μmol·m-2·s-1 for 16 hours/day leading to
a daily light integral of 15 mol·m-2·d-1.
Plants should be watered every two days
with a complete nutrient solution at
100-ppm nitrogen, until the grafting date
which is about 20 days after sowing the
scion cultivar (Figure 1).
Tip #1: Start with good seed. Poor
quality seed can lead to bad germination,
increased risk of disease and inconsistent
seedling growth - all of which will make
grafting much more difficult.
Tip #2: Always run a small trial before
using a cultivar for the first time. Sowing
a few seeds and growing them to grafting
size can help to determine the cultivar's
germination rate, growth rate and
consistency.
Tip #3: Seed the rootstock variety two
days after the scion variety. Rootstock
cultivars have vigorous growth and,
therefore, grow faster and reach sufficient
stem diameter for grafting earlier than
scion varieties.
Phase 2: Grafting
Twenty days after sowing the scion
cultivar the seedlings of both the scion
plants and rootstock plants should have
a stem diameter of approximately 2
mm (although there will likely be some
variation) and are now ready to graft.
While it is possible to graft older plants,
younger plants heal faster and have a
higher likelihood of surviving.
The first step in the grafting phase is
to choose a scion plant and a rootstock
plant with compatible stem diameters
(Figure 2). It is important to graft plants
with very similar stem diameters to
achieve the best success. Stem diameter
measurements should be taken below
the cotyledons for both the scion and
rootstock plants.
Figure 2. A rootstock, and scion, right,
seedling with compatible stem diameters
for grafting.
The next step is to cut the rootstock
below the cotyledons. Cutting below
the cotyledons eliminates advantageous
shoots from growing out of the rootstock.
The cut can be done using a razor blade
at a 45- to 60-degree angle with a deeper
angle allowing for greater surface for the
graft to take hold. Then, a transparent
silicon channel grafting clip should be
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December 2020

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