Fertilizer Placement Affects Weed Germination and Crop Growth
There are three methods used to apply controlled release
fertilizers (CRFs) to container crops: topdressing, incorporating, and
dibbling. Currently, topdressing and incorporating fertilizers
are the most common methods. Dibbling fell out of favor with fertilizer
manufacturers and growers due to concerns of plant injury with roots
being so close to the critical mass of fertilizer.
Dibbling use to be a common and recommended practice. Sierra
Chemical (now Scotts) use to provide growers with a technical bulletin
called "Dibble Application of Osmocote or High-N CRFs (Bulletin #30)".
When a few cases of injury occurred because of dibbling fertilizers,
manufacturers and subsequently growers stopped dibbling fertilizers completely.
This occurred despite the fact that it was a very successful fertilization
method that generally resulted in better crop growth with lower fertilizer
rates and less nitrogen leaching compared to topdressing and incorporating
Work by Sven Svenson demonstrated that incorporating fertilizers
resulted in fewer liverwort infestations compared to topdressing. He
determined that liverwort thrive in high N and P environments, and by
incorporating fertilizers you remove most (though not all) of the N and
P from the container surface. Perhaps dibbling fertilizers would
even further reduce available N and P and further reduce liverwort growth.
A Few Mistakes That Prove A Point
First, my technician wanted to find a better way to collect
weed seed (we collect weed seed regularly to use in experiments, I know
that sounds weird). She used a method of floating seed as a way
to separate the seed from other debris. To compare her method to
mine, she placed some seed in containers to test seed viability. She
filled up one gallon containers with 100% Douglas fir bark amended with
micronutrients, then overseeded the bittercress. I told her to topdress
with Osmocote fertilizer, but she didn't listen. Within 1 week all
the seed germinated........ but 2.5 months later, the weeds were still
at the cotyledon stage. The photo to the right is a picture of the
bittercress seed taken 2.5 months after germination. Without fertilizer
(N, P, and K), the weeds appeared to have germinated just fine, but failed
to grow beyond the cotyledon stage. Note that there was sufficient
micronutrients available to the seedlings, they were only lacking N, P,
A second mistake occurred. I decided to put together
a demonstration for our September field day (2002) that evaluated all
the common preemergence herbicides used in container production. We
potted up 3 species, topdressed them with 12 g of Osmocote 18-6-12, overseeded
them with either bittercress, oxalis, or annual bluegrass, and then
applied to each container one of 14 herbicides. By 45 days after
herbicide application, no herbicide provided control of any of the weed
species. Most herbicides should provide control for at least 80
days, but at 45 days all had failed, including our most commonly
used products at the maximum recommended rate (Rout, OH2, Snapshot, Regal
O-O, Ronstar, and others).
We decided to test these ideas with a controlled experiment. In
all, we conducted 5 experiments which all had the same general results.
Below is a description of one of the experiments performed by our
summer intern, Kathy Von Arx.
We used the follow experimental setup:
- Lavandula x intermedia 'Gross' were potted
into one gallon containers.
- Pots were overseeded with 15 oxalis seed each.
- Plants were grown with overhead irrigation on a container
yard and evaluated for 4 months.
We applied the following herbicide and fertilizer treatments,
each to 10 containers:
|Snapshot rate (lb/acre)
We used a maximum of 150 lbs of Snapshot because the label recommends
this rate for oxalis control (the species we overseeded with). We
also used 1/2 of that rate and no herbicide to see if fertilizer placement
along with low or no herbicide could be used to provide adequate oxalis
Weed growth was dramatically improved by dibbling fertilizer (Chart 1). Even at the recommended
herbicide rate (150 lb/acre Snapshot), weed control was improved by dibbling
the fertilizer instead of topdressing. Most surprising was the high
level of weed control obtained when no herbicide was used. With
no herbicide, topdressing and incorporating fertilizers resulted in about
60% weed control (not commercially acceptable) while dibbling fertilizers
resulted in over 95% weed control.
Growth of lavender was also affected by fertilizer
placement (Chart 2). Regardless
of herbicide rate used, dibbling fertilizer resulted in larger plants
compared to topdressing or incorporating fertilizers. The authors
noticed at the beginning of the experiment that plants with dibbled and
topdressed fertilizers began growing more quickly than those with incorporated.
This was likely the result of an available supply of nutrients in
containers that were topdressed or dibbled, and a more diluted supply
of nutrients in containers where fertilizers were incorporated.
Despite the result of larger plants with dibbling,
roots of plants that were dibbled were smaller than those that were incorporated
and slightly smaller than those that were topdressed (Chart 3). Root evaluations were done by
subjectively rating the % of the rootball-container interface that were
covered by roots. This may not be the best measure of root growth,
but it does serve as a good indication if a production practice
influences root growth.
While root ratings were lower for dibbled plants where, it
was noted that all plants had root systems that would allow them to be
- Dibbling fertilizers resulted in superior weed control and
shoot growth, but slightly reduced root growth.
- Incorporating fertilizers resulted in moderate weed control,
poor shoot growth, but excellent root growth.
- Topdressing fertilizers resulted in poor weed control, excellent
shoot growth, and slightly reduced root growth.
If you are considering dibbling fertilizers to improve your weed
management program, consider the following suggestions:
- Use only CRFs rated to release nutrients for 8 to 9 months
or longer (do not use 3 to 4 month products).
- Until we have more research, only dibble fertilizers when
potting in the spring or early summer. Past experience by some
growers indicates there may be a potential for salt problems during a
cold dry winter.
- Conduct small trials to make sure your crops are compatible
with this method of fertilization. Some species may be sensitive
to having fertilizers so close to their root system (though I believe most
will grow fine).
With proper irrigation management (next year's research) and proper
fertilization management, weed control can be drastically improved without
changing your current herbicide program.
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