Picture of Grow a square watermelon
This is really, really easy to do. (Note: you may be able to use this technique to grow other stuff in the form of a square ( actually in the form of a cube) and apply the general idea to other shapes.)

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Step 1: Materials and tools

Picture of Materials and tools
You need 6, 8" square, 3/8" thick (or thicker) sheets of polycarbonate plastic. (Lexan)

You will need 4 gate hinges and 2 hinged clasps with flat head machine screws and hex nuts plus at least 8 or more 1" to 1-1/2" long, thin wood screws and possibly a 36" length of angle iron or aluminum angle (cut to 8" lengths to yield four pieces) if you have thinner polycarbonate, a power drill and hole saw, and a hand saw or power saw such as a jig saw.

Step 2: Cut the stem pass through

Picture of Cut the stem pass through
In the first sheet drill a 1" diameter hole in the center with the drill and hole saw. Then, using a hand or power saw, cut 2 parallel lines from the hole perpendicular to one edge of the sheet and remove the material between.

Step 3: Add first set of hinges

Picture of Add first set of hinges
flat head machine screw.JPG
Use a box to rest the left side sheet, place the top sheet left edge over the left side sheet edge and fasten the sheets together as shown in the diagram with 2 hinges.

Step 4: Trim the inset sides

Picture of Trim the inset sides
Measure the width of the sheets and trim each side of the the inset sides by this width.

Step 5: Fasten sides

Picture of Fasten sides
The edges of the inset sides will have to be trimmed the thickness of the poly you use so they can be inset and still keep the box square. Trimming these edges is probably the hardest step.

Drill pilot holes through edge of the left and right sheets into side sheets between them to accept the wood screws. Use 2 or more screws per joined edge. A piece of angle iron or aluminum can be used to beef up the corners if the polycarbonate you have is too thin. (If the poly is too thin, however, it may bulge.)

Step 6: Do the bottom

Picture of Do the bottom
Place the box on its top, add the last sheet for the bottom and secure with hinges as you did the top.

Step 7: Add the clasps

Picture of Add the clasps
Clasps go opposite the hinges on the top and bottom sheets.

Step 8: Usage

Picture of Usage
Watermelon square.JPG
Plant watermelons. As soon they begin to grow large enough to place them in the box with the stem through the slit and hole, do so. Remove when ripe.

Step 9: Addendum: How to grow a watermelon

Picture of Addendum: How to grow a watermelon
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How to plant your watermelon

Watermelons come in about 1,200 varieties worldwide, divided according to season and seed production. China is by far the worlds largest grower of watermelon. Growing time is from 70 to 85 days for all varieties and growing season begins when all danger of frost has past. Watermelons can be grown indoors anytime of year but require extra amounts of space and constant temperature between 80 and 85 deg. F.

Seeds for seedlings to be transplanted outdoors may be started indoors 3 weeks before transplanting outdoors. Seeds are planted one inch deep in groups of 3 and seedlings thinned to the best 1 or 2 for planting. You can use this method with peat or potting soil to start seeds indoors. Seedless varieties must be planted alongside seed producing varieties in order to pollinate and set the fruit.

Hills for single transplants are spaced 2 to 3 feet apart while double transplants are spaced 4 to 5 feet apart. Rows are spaced 7 to 10 feet apart.

How to tell when your watermelon is ripe

Use black plastic to cover the hills and rows and plant through the plastic. Drip irrigation is sometimes used sparingly in cooler climates.

Use a combination of the following indicators to determine when your watermelons are ripe:

1. light green, curly tendrils on the stem near the point of attachment of the melon usually turn brown and dry;
2. the surface color of the fruit turns dull;
3. the skin becomes resistant to penetration by the thumbnail and is rough to the touch; and
4. the bottom of the melon (where it lies on the soil) turns from light green to a yellowish color.

Many watermelons do not emit the proverbial "dull thud"when ripe. For these, the dull thud may indicate an over-ripe, mushy melon. The above indicators for choosing a ripe watermelon are therefore much more reliable than "thumping" the melon with a knuckle.

Source: University of Illinois Extension Service

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jeffcole3 months ago

They look like minecraft fruit :)

kcls5 years ago
I can see it now- instead of a watermelon, grow a pumpkin in a cube shape. Then, carve a Domo-Kun face on it. Voila! Domo-Kun Jack-O-Lantern.

I've got dibs on the 'ible.
melonmoldcom kcls4 months ago

We did pumpkin last summer/fall. They turned out great!

Best Jack O latern on the block!


omnibot kcls5 years ago
Do it!
kcls omnibot5 years ago
Gotta wait until I have a garden to grow it in! Living on a boat and all, I don't have a very big yard :D

You just have to be imaginitive about where to grow. I have seen people grow things on walls and roofs. Doing this also cuts cooling and heating costs.

you don't need a garden i grew a pumpkin in a large pot and trained the vine to a railing. when fruit formed i used pantyhose to support the weight.
you do, it's just REALLY flooded
So... rice?
aseaheru xenor4 years ago
salt water.......o_0
put it on the mast head. o_0
Hiroak kcls5 years ago
I'm on a boat Motherfu Ha ha very orginal I know I lived on a boat, the sail type for 2 years and build floating garden boxes out of blue foam board. I had a slip larger than my boat so I just attached them to the dock. Then I got a bigger boat and build two above ground planter boxes on the deck. I even tried to grow an orange tree, to combat the scurvy but I live in Alaska.
huh ... seems many guys are really interested in the pumpkin .....why don't we just make a mold for the pumpkin ,but what shape you want it to be ??
melonmoldcom made it!4 months ago

It's that time of the year again! Get your plastic molds now at melonmold dot com!

Great summer project and fun for the while family! Grow a watermelon or pumpkin into a square or heart!

MelonMold dot com

davidbarcomb8 months ago

I saw this in a Japanese show and it was really nice. Great instructable

scott.gosa1 year ago

$80 for materials? Geez!

For those of you lacking either the time or tools to complete this task, we offer a commercial box mold specifically for molding watermelon at MelonMold.com. We're inexpensive and ship internationally from the US.



The actual size of the melon your mold will produce
is 7.5” at its tallest point and 7.5” at its widest. It is 7.5” at its
deepest point.


Does the Box have to clear and made out of plastic?
it is better be clear cos the fruits need sun to grow to look better , and plastic seems to be the best choice we have by now ..

I read the other day about a japanese guy who managed to make a hearts shaped watermelon 'after many years of research'. Then I already knew it was bogus as it is just a matter of the right mould and here I see those are available already.


fashboy1 year ago

'This is really, really easy to do.' OMFG SO EASY

i have tried this method ,but seems not easy .....we have grown some like the below one :

Suppliers for plastic square box mould for square watermelon.jpg
it is great job to make this square watermelon box ,but very time consuming ...
why don't you just come and buy one very easy ,it just cost you like $10 , will save you a lot of energy .for anyone interested , come to www.fruitmould.com
oh ,by the way , i have a client grown a lot of square shape watermelons , he got many experience on this ..check the photo
i bet you don't need all the hardware. you can probably use big rubber bands to hold the form in place.
good request , but you are really underestimate the growing stress from watermelon , it is a big guy :-)
Would a box work? I do not have the ability to make your cool invention
if your box is strong enough ,then it should work ,yes, to make one mold is really practical skill...
elkrivermr2 years ago
how do you deal with rain and condensation?
rain will not affect the growing ,because even with some rain store inside of the molds ,it will be gone soon by the sun shinning ,isn't it ?
sinnae1 year ago
I feel like this would introduce a ton of moisture-induced rot unless plenty of ventilation is drilled into each panel.
yes , you are right , it has 4 holes on the size wall of the molds to keep ventilation ,the sun shinning might be really strong , you need to cover the black net at the beginning ...
Not4Guy1 year ago
I wonder if this could be doing to shape a pumpkin into a skull for witches hat?
this polycarbonate plastic made square mold can be used on pumpkin ,but it is really hard to say if can success , depends on the mold thickness and the pumpkin stress ,ours are 3mm thickness ,should be strong enough for pumpkin , go www.fruitmould.com , check more shaped fruits ,you will be amazed for sure ...
OH, this polycarbonate plastic made square mold looks good ,but we also have another injection made square mold for watermelon which is well tested in past few years,check www.fruitmould.com , you will find more amazing shaped fruits ...great fun :-)
Nukeman3 years ago
The Lexan to build this costs about $80. Will plywood work?
myakka3 years ago
How about triangular( pyramid) shaped ones. Or dodecahedrons!
myakka myakka3 years ago
Or how about make the entire alphabet
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