I've grown numerous "large" pumpkins here in the US, and can tell you
that there are several factors at play in growing really big ones. One
is to prepare your garden bed with well rotted manure, (I use horse or
cow), and do so deeply. Pumpkin roots run deep and the plants feed
heavily. Make a "hill" in to which you would deposited four to six of
your seeds, covering them to a depth of about 1".
At the three week mark after germination, select the two largest plants,
and snip off every other plant. From these two, allow them plenty of
space to grow, caring not to snip or prune anything just yet. The goal
is to get the largest, more robust vines possible, as large fruit rarely
grow on wispy vines.
Once you begin to see the female flowers forming, select the two
healthiest looking for your project. Upon their opening, hand pollinate
both, using a cotton swab and transferring pollen from the male flowers
to the female stigma. Pumpkin size is directly linked to the level of
pollination achieved, so don't leave it to chance/bees.
After you've noticed that the pollination has taken, (usually two weeks
to be certain), begin to remove all subsequent male and female flowers,
along with any developing side branches. This will divert all energies
in to the formation of the pumpkins. Given a good warm summer with
plenty of rainfall, one should be able to achieve a fairly good sized
I have a small amount of experience with this. Nearly 25 years ago,
I grew several 'Atlantic Giant' pumpkins. I put lots of composted (and
a small amount of fresh) horse manure in a deep hole under the plants.
I also watered regularly, and thinned the pumpkins once there were 2 or
3 to a plant.
Especially important is mulching around the plant, but you don't want
to put straw right up where the leaves join the plant. The plant will
put out additional root systems at these areas which will help give
additional moisture and nutrients to the plant. It will also help
anchor the plant against damaging winds.
I ended up with a 263 pound pumpkin, which won 1st at the county
fair, and would have taken 1st at state if I had been able to get it
there that year. I've always wanted to grow another huge pumpkin. From
what I've read, these are actually squash, but most folks treat them as
If I tried it again, I would use drip irrigation and a timer, to make
sure the plant got optimum amounts of water.
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