living mulch - do pumpkins shade the soil

Quite often I have read about the "three sisters" method of planting.  People state different reasons for each of the three plants being planted together.  More often that not people state that pumpkins are used as they shade the soil, and provide a living mulch.  But is this really the case?

First, let me say that the three sisters method works well when done properly.

If you grow field corn, dry beans, and pumpkins together they do crop well together.  The beans sequester atmospheric nitrogen that the corn appreciates, the corn provides a stalk for beans to grow up but more importantly the corn exude sugars into the soil for the pumpkins, the pumpkins are meant to shade the soil, but they don't.  Or at least they don't shade the soil when it is hot.

If you have ever stopped to look at pumpkin leaves on a cool day they are large and cast an impressive shadow.  If you look at them during any kind of heat you may notice that they go flacid to conserve water.  When they are flacid they do not cast much of a shadow and do little to cool the soil.

When we lived in Central West NSW we had about 9 weeks each year where the day time temperature exceeded 40 C.  It was incredibly hot and dry for a very long time.  Pumpkins were all but worthless for providing shade in that kind of heat.  The leaves would go flacid at about 7:30 each morning and stay that way until they were watered in the evening.  It kept them alive, I got a crop out of them, but didn't help the corn.

I know that people will argue with me on this, so I took some pictures to demonstrate my point.  I took these pictures last summer on a day that reached 30 C.  It was a still day with no wind.  If pumpkins don't cast shade on a day like that they are not going to do a great deal in any real heat or if it is windy.

Pumpkin leaves in any heat wilt badly, but do they still cast much shadow?

Pumpkins from this angle you can't tell if they shade the soil

Pumpkin leaves, look how little shadow they cast at 30 C

I have had people argue this point with me and say that in the three sisters method that there is plenty of shade on the soil.  This is partly true.  In the three sisters method there is often plenty of shade on the soil, but it is not due to the pumpkin leaves.  Look at the picture above, pumpkin leaves don't cast much shade at all when the days warm up.  They cast even less shade when it is hot.

I think the shade that may be due to the corn.  So I took some pictures of the soil under my corn plants.  It was the same day as I took the pumpkin pictures, it only reached 30 C and there was no wind.  On hotter days the shade under corn plants is much the same as their leaves curl but don't wither away like a pumpkin.

Corn plants (on the left) cast more share than pumpkins (ignore the potatoes on the right)

The three sisters method works well when done properly, but what about when you want to shade soil under other tall crops such as yacon?  If you want a plant to shade the soil what should you use?  As you can see in the pictures above pumpkins are not very good at shading the soil unless the day is cool and still.

In the past I have attempted to use things such as clover.  This is not great, it survives cool weather and appears to die in the heat.  No matter how much water I gave it the clover always died in Central West NSW over summer.  I need something that stands up to the heat of summer, grows densely to shade the soil, but does not get too tall.

I have successfully used purslane to shade the soil under corn and a few other crops.  This worked perfectly, it survived the heat, if watered it produced a thick deep mat, it did not appear to compete with the corn, it self seeded and dies at the end of summer.  It is edible by people as well as other animals.  Purslane apparently contains highest amounts of omega 3 fatty acid of any plant we know of and is very healthy for poultry.

Some people view purslane as a weed, and it self seeded so I more encouraged it rather than controlled where and when it would grow, so I looked for other options too.

I trialed strawberries as a ground cover to shade the soil.  I grew an alpine strawberry that produces a lot of runners.  This produced a thick ground cover that had the benefit of producing delicious edible strawberries.  The strawberries are a short plant, have a relatively shallow root system, and do not appear to compete very much with taller things.

Yacon with strawberry ground cover so dense the soil is completely shaded

Strawberries shade the soil even on hot days - note the strawberries ripening in the background

They also protect the roots from frosts a little

Growing strawberries as a ground cover worked well during the growing season.  Strawberries need a bit of water to survive, but so did the yacon so it wasn't a problem.

When it came time to dig the yacon it was less than ideal as I also had to dig the strawberries.  The strawberries are pretty hardy and survived being dug pretty well so it wasn't too much of an issue.

I do wonder if I could grow something like a creeping thyme as a ground cover.  I may try a few different things and see how they work.