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San Diego

So, what causes a concavity on the bottom of the pumpkin? I've only grown a few pumpkins in the short time I've been in this hobby, but all have been concave on the side against the ground. I've tried putting the pumpkins on sand, mill fabric, and sand + mill fabric.
Some thoughts, suggestions:
1. this is an adaptation to being large... like thickened walls and a squat shape, to support the huge weight of the kin.
2. This isn't an advantage for the pumpkin, but does happen in response to huge weight, e.g. because of the thick walls.
3. Genetics: some plants are more apt to have this feature. I happened to have grown some of these.
4. Nutrients not at the right ratio/level, or stunted growth for some other reason.
5. I haven't yet discovered the right combination of materials to put under the growing pumpkin.
6. Some combination of most of these factors.

7/30/2014 12:58:39 AM

Lodi, California

Kevin,

I think that it has to do with the friction on the bottom of the pumpkin pushing the centre up slowly but surely. This would be reduced by sand, but not entirely.

7/30/2014 2:12:26 AM

Lodi, California

So as the pumpkin grows outwards, pressure builds up under the pumpkin, and the easiest way for it to grow in concave into the pumpkin.

7/30/2014 2:13:47 AM

Muddy Waters

I dont think it affects weights much as the thickness appears to be the same all the way.Weather flat or curved the fruit mass is equal.They seem to be very mild on my kins,using cement board,delta sub floor & sand .

7/30/2014 5:24:40 AM

Menomonie, WI (mail@gr8pumpkin.net)

I think it is because they are pumpkins...and a pumpkin naturally wants as little pumpkin surface touching the ground. Even field pumpkins can get concave sides.

7/30/2014 7:31:46 AM

Central Illinois

it might help to stop a concave in the top that would fill with water, maybe its what they always taught in school,an arch is stronger than being straight

7/30/2014 7:59:48 AM

southwest MO

Maybe the pumpkin does this intentionally to help strengthen it's self. Like a punt in the bottom of a wine bottle?

7/30/2014 8:46:19 AM

Springfield, VT

It is due to friction under the pumpkin, flat bottoms are from low friction at the bottom of the fruit. Without the ability to slide the pushup inside.

David Hu did a study and his results are on the web, a search will bring up several links.
Take a look at this link
http://www.wired.com/2010/10/big-pumpkin-physics/

7/30/2014 8:56:54 AM

BC, Canada

Has anyone ever used one of those inflatable rings under the pumpkin? The blow up type that are used to ease pressure on the dark side, or maybe one of those kiddie swimming rings. Small water bed?
I always thought the shape was an effect of the physics as Baitman said. The weight from the top and sides is sliding down the edges like two hands squeezing a balloon and that causes the lower middle to push up. Or that's what I thought was happening.
Good article VTWilbur.

7/30/2014 1:48:10 PM

San Diego

Cool article, thanks for the replies! Good to know that the weight may not be compromised even if a concavity develops.

7/30/2014 2:31:31 PM

Joliet, IL

i have read the study. i also have had both flat bottoms and concave bottoms. even those who use sand will have some more concave than others. personally i believe its a structural occurance that happens with fast growing yet semi rigid live materials that change shape as gravity, form, and shape play factors in development. the wine bottle punt or reversed arc comments seem most accurate coming from someone working in the engineering field. point loading and pressure will find ways to fight gravity as weights and lateral pressures increase. just my 2 cents.

7/30/2014 5:26:49 PM

Colorado

What shazzy said. I see no consistent correlation between the chart and flat or concave bottoms. A concave bottom wants to grow that way with or without sand. Its complicated.

7/30/2014 6:34:25 PM

Joliet, IL

anyone who has seen "the bean" in millenium park in chicago can take that mental image and cross with a waterballoon and understand point loading stresses from gravity and shape. the concave bottom of the bean bears more concentrated point load pressure.....so widen the bean and you will have a stronger structure laterally then if the bean was flush to the ground. again just my 2 cents and would love input from those with sand and mill fabric as opposed to myself and those who go mill fabric only.

7/30/2014 6:40:25 PM

Joliet, IL

and personally the only bottom splits i ever had came not from the concave bottom.....it came from a internal fracture leading out and runs thin on skin....like a top dill ring rib split hole but on the bottom. but you dont see that until you harvest your seeds and look at every square inch of the internal cavity to stay away with the crystal calcified ball imternal split gene....lol

7/30/2014 7:02:20 PM

Joliet, IL

internal....doh

7/30/2014 7:03:24 PM