An Introduction to the Process of Grafting in Greenhouse
Tomatoes- Part 2
By Natalie Bumgarner, Horticulturist
CropKing, Inc. Lodi OH
So, what comes
after the graft?
In the last blog
post, we went through a brief photo tour of the first two steps in the grafting
process (1. Setting the Stage, and 2.
Doing the Grafting). However, when discussing grafting, it is important to know
that preparation and grafting are only the first two crucial steps. Graft
healing is that key third step in producing successful grafted tomatoes for
your greenhouse. So, as the third blog in this series on grafting processes, I
want to focus on methods of healing and the transition process from grafting to
transplanting in greenhouse tomatoes.
The Healing
Environment
It is important to understand
that there are three crucial environmental factors involved in graft healing-
humidity, light and temperature. Of course, these are the same key
environmental elements that we all manage in our greenhouses, but the specific
levels and ranges differ for graft healing versus production conditions.
- Relative Humidity- Begin around 95% immediately after grafting and reduce as healing progresses.
- Temperature- 80 to 84 °F is optimum
- Light- Begin with essentially no light for the first day or two after grafting and then gradually increase to light in the range of 500- 700 fc or around 100 µmol/m2/sec.
The Healing
Chamber- Location
When carrying out
grafting, there are two kinds of locations typically used by growers. The first
is a chamber where conditions can be closely maintained to optimize healing.
While useful for some propagation or research facilities, most growers will not
have access to dedicated growth chambers and will need to use their greenhouse
or attached buildings. Graft healing in greenhouses has been done for many
years, and small to mid-scale temporary structures can certainly provide you
with adequate healing capacity if managed well.
The Healing
Chamber- Design
Most commonly,
healing chambers or tents are created with clear plastic within the larger
greenhouse space. In these tents, humidity can be increased and maintained with
misting either by hand or with an automatic mist system. Temperature and light
also need to be carefully monitored and managed. Shade cloth in different
weights or layers can be used to initially exclude light and then gradually
increase light during healing. Be very careful to monitor temperatures because
these clear plastic tents- even shaded- can heat up quickly in a greenhouse due
to solar gain. This increased temperature can in itself be high enough to
damage seedlings, but it can also quickly lower the relative humidity and
desiccate young grafted plants. For more northern growers seeding and grafting
in early winter, greenhouse healing chambers can typically be managed
adequately (as illustrated in this blog). However, if grafting in the mid-to
late spring or summer, maintaining appropriate temperatures in a tent in a
greenhouse may become difficult due to ambient light and heat. Under these conditions, you may need to use
your head house or some other work area fitted with a temporary chamber and
lighting.
These two images
show a small healing chamber constructed within a larger greenhouse (top). PVC
pipe was used to construct a small tent first covered and lined with clear
polyethylene plastic (4 mil construction plastic is fine) to allow a fairly
tightly sealed airspace. A dome over the tray can also be used for smaller
numbers of plants (bottom). Then shade cloth in one to two layers is used to
cover the tent or dome.
Healing- The First
Few Days
For the first day
or two after grafting, near darkness is needed. This can be accomplished by
covering with double or triple layers of shade cloth (depending on whether 50
or 70% shade cloth is used). Humidity is maintained with misting, but be sure
to mist the plastic walls of the chamber or dome rather than the plants
themselves. We want moist air to induce healing and not wet plants to encourage
plant decay.
Transitioning Back
to the Greenhouse
The goal of graft
healing is to allow the scion and rootstock to fuse together and form new
vascular connections by providing low stress conditions. Then after connections
are formed, the plants is gently brought back to typical production conditions.
So, humidity levels start out around 95% and after ~5 to 7 days drier air is
slowly introduced to the crop by increasing venting. Likewise, light is slowly
increased by removing shade and/or increasing supplemental lighting and
temperature is slowly reduced. Grafted tomatoes should be ready to transition
back to the greenhouse in a little under or a little over a week depending a
bit on the crop and environment.
In transitioning
young grafted plants back to typical greenhouse environments, conditions are
slowly changed and plants are closely monitored for wilting or signs of stress.
If plants show signs of stress, you can increase humidity or reduce light again
for a day or two. However, keeping plants in the high humidity healing
environment too long can induce roots from the scion (adventitious). These
roots will hinder proper graft healing. Eventually, a ‘moment of truth’ will
have to be faced. Grafts that fail to heal after 7 to 14 days are unlikely to
produce high quality grafted plants due to a variety of factors including
improper scion/rootstock size matching, graft technique, genetic
incompatibility, or healing environment.
Newly grafted and
healed plants transitioned to the greenhouse and awaiting transplant.
These are close up
images of graft unions approximately eight (L) and six (R) days after grafting.
The plants have both already been transitioned back to normal greenhouse
conditions.
Remember, no one
achieves 100% success…
An unsuccessful
graft where the scion has produced adventitious roots to absorb water from the
high humidity air in the healing chamber. As the wilted scion illustrates,
these roots hinder graft healing and the plant often dies under greenhouse
conditions.
Tomatoes being
transitioned after healing. Some are showing slight wilting, but have adequate
grafts to sustain them. A few, though, are wilted down and will not survive.
Successful grafting is based on skill in grafting and care in healing- both of
which generally improve with practice!
Planting in the
Greenhouse
These two images
show tomato plant graft unions approximately two weeks after grafting as they
are being planted in the Bato buckets (L)
and a plant growing in the bucket approximately six weeks after grafting
and four weeks after transplanting (R).
This blog completes
our introduction to grafting in the greenhouse, but we will continue to provide
updates on the growth of the plants and other facets of using grafted plants in
small to mid-scale hydroponic greenhouses.
