Friday, March 28, 2014

Huge Harvest of Pawpaw/Papaya in cold climate

Huge harvest of pawpaw/papaya-like fruit in a cool climate.

As of Nov. 2013 my wife and I had four adult Babaco “trees” (Carica pentagona, a kind of cold-tolerant pawpaw or papaya)  growing inside an unheated plastic greenhouse that leans against our sun-facing back wall.  It is about 2.5 metres wide by 4 metres long.  Say ten square metres. The babacos’ leaves occupy most of the top of the greenhouse, but there is also a small avocado, a fruiting Ficus coronata, pepino bushes, a tamarillo, and passionfruit.


 Inside the greenhouse the trees are growing in just 6 inches (15 cm) of rich soil, over an old concrete patio.

 The babacos mature a few fruits during the winter, when they are fairly stationary. Then as the Spring weather warmed up from the start of September 2013 we had one or more fruits, weighing about 0.7 kilos (that’s about a pound and a half for those still using the old imperial units), maturing each week. 

 The babaco is a theoretically perfect fruit, like strawberry, in that there is no skin or seeds to remove. One eats the whole fruit.

 I particularly like to eat one uncooked, simply cut into quarters, dabbed with apple juice and stored in the fridge.  My wife Jan prefers them baked for 30 minutes at 180 Celsius, with a little brown sugar.  A Bhutanese friend chops them up with cheese.

 This rate of fruit production from the 4 plants accelerated in October until by late November 2013 we had consumed or given away over a dozen large fruits, each around 700 gm. We also had a table covered with  a further 10 large ripe fruits, plus 8 that had to be picked green when a branch broke. (These were used as vegetables, e.g. in curries, rather than as fruit.) There were a further 14 advanced fruits maturing on the “trees” to ripen over summer.   Thereafter there was a hiatus in the fruiting till some of the smaller fruits came on, and till three smaller trees become adult.  (With better management it should be possible to have babacos  fruiting continuously.)

 When I weighed these 18 fruits that we currently had in the house, they totalled over 10 kilos. Luckily the fruits store for a month or more at room temperature, provided you pick them before they are uniformly yellow.


 Cultivation: How we grow babacos in a cold climate.
 Australia’s proverbially chilly capital, Canberra, might seem an unlikely place to be reaping large harvests of a close relative of the tropical papaya or pawpaw, Carica papaya.  Yet we have found it surprisingly easy to do. Here’s how.

The flowers require no pollination, and the fruits, which set automatically, contain no seeds. All parts of the fruit are edible.

Canberra, at 600 metres altitude, in the foothills of the Snowy Mountains, gets about 100 frosts a year and is not reliably frost-free until mid-November (the last month of Spring). Frosts commonly return in May. The lowest recorded temperature is minus 10 Celsius.

Such a climate rules out growing the true pawpaw/papaya outdoors. However its equally bountiful sub-tropical relative the babaco, Carica pentagona, is much more tolerant of persistently cool weather. The problem is that while babacos tolerate sub-zero temperatures, perhaps even down to minus 6 Celsius, they dislike frost on their leaves.   (Hence the greenhouse!)

 As best I understand it, the problem with frost is that if a dry breeze moves over a frosted leaf at dawn the frost can “sublime”, as chemists say. This means that it evaporates directly from the solid to the gaseous state. In the process it may drop the temperature of the leaf it encases to temperatures well below the ambient air temperature.  An alternative explanation, favoured by the Wikipedia article on Frost, is that "In the absence of a site nucleating the formation of ice crystals, leaves remain in a supercooled liquid state, safely reaching temperatures of −4 to −12 °C (25 to 10 °F). However, once frost forms, the leaf cells may be damaged by sharp ice crystals."

 The obvious solution is to grow babacos under the eaves of a house , and perhaps protected by a simple sheet of plastic hanging down from the eaves. That way dew is less likely to get in, and frost unlikely to form on the leaves.  I have found this worked well with strawberry guavas, but less well with babacos. For babacos, it seemed, some kind of minimal greenhouse was needed.  

The door, here shown open, is simply a door-shaped sheet of transparent vinyl held taut by the weight of a thin plank stapled to its lower end. One rolls it up, with the plank, much like a blind. Black Velcro patches help hold it in place when closed.

I also had to solve the problem of drainage. Babacos need better drainage than our local clayey soil provides.  Growing them in very large pots was one solution, and this also made it possible to lug the plants inside and treat them as (rather attractive) ornamental pot-plants during the coldest part of winter. However by the time they are 3 metres high, babacos need very big pots. Also, potting-mix breaks down within two years. It compacts, and ceases to drain reliably. So the replacement of potting-mix for such large plants became a chore.

 Fate provided a single solution to both problems. To explain it, let me describe our back garden. It slopes downhill North-Eastward, into the rising sun.  This is the perfect facing for a Southern-hemisphere garden, since the slope also makes the morning sun’s rays hit the garden more directly. The plants warm up quickly in the morning, and reach their ‘operating temperature’ hours earlier than would happen on South-Westerly slope. At the same time, the excessively strong afternoon sun is mitigated because the rays are by then striking the garden somewhat obliquely. As a further blessing, the bulk of the house (plus a slight hill behind it) blocks out the cold winds from the South and West.
 The warmest and most sheltered part of the back garden was therefore right against the back wall of the house, where the North-East-facing house-wall reflected the morning sun’s heat back into the garden. (There is of course a similar warm spot in most home gardens, against a sun-facing wall.)
 And within this wall there was a sheltered rectangular area, framed on one side by a bedroom that juts outs, and on the other by a flight of brick steps going down from the back door.  This should have been the choicest area for planting into—except that the previous owners had concreted it, turning it into a patio.

The greenhouse in situ. The brick steps are just out of view in the foreground, with a double safety-rail at the top of them.
I had long felt that this concrete patio at our back door was a wasted area. Unable to plant into it, I used the patio as a space to put a few favoured pot plants—until one day I had a brainwave.

 I laid a single row of bricks on their side about 2 metres out from the wall, then bought ten bags of potting-mix (on special at the supermarket) and filled in the rectangular area between the wall and the bricks with potting-mix. Obviously the depth of soil over the cement was only a few centimeters—the height of a brick on its side. Yet I planted out this area as if it was a garden bed. (I had read that Parisian market-gardeners used to boast that they could grow perfect vegetables upon cement. All they needed was a few centimeters of well-rotted horse-dung over the concrete.)

 The plant growth that followed was magical, perhaps because the slight slope in the concrete patio underneath guaranteed good drainage. (Patios are normally built with a slight slope so that they drain). As well, the shallow soil guaranteed aeration. Also, although the soil was on the acid side, plants could still get access to lime and more alkaline conditions from the concrete below. 

 Despite the shallowness of the soil, this patch did not seem to need much more watering than any other bed—though it’s true that I do water it at least every 2 days  in summer. (There is a hole cut in the side of the roof’s spouting that drops water directly into it; so there is no need to water it in weeks when there has been rain.)

 Later I got this whole area covered with a plastic roof, albeit with a fair bit of ventilation, and with transparent plastic side-walls.  It was a very cheap lean-to greenhouse, with slits in the plastic roof so that rain-water fell through. The existing house walls provided two sides of the rectangle, and bamboo poles, grown in our garden, held up the other two sides and the roof. The transparent plastic was free. It had been wrapped around a new lounge suite. (It worked beautifully for one summer. Then, since it was not UV stabilised, it fell apart and had to be replaced.)

A new and much better structure replaced it, with poly-piping rather than bamboo supports. My friend Bernard Davis built it, and designed several innovative features that are shown in the photos. These include a roof that can be partly rolled up in summer, since Canberra gets the occasional day of 40 degrees Celsius, which would otherwise cook the plants. The total cost, including labor, was about $900.  The roof is standard translucent greenhouse plastic (polyweave). This has the advantage of distributing the sun's rays more evenly below. (In the photos it looks solid white, but most of the sunlight passes through it.) However a greenhouse made entirely of this material would look like a white barn, and the plants inside it would be invisible from outside—destroying the magic of the enclosed space.

I found it surprisingly difficult to buy transparent greenhouse plastic for the walls. Suppliers shook their heads, or assured me there was no such thing. Eventually I bought what Bunnings call “table-top vinyl” from their Flooring Section. This was available in four thicknesses. The thinnest was not UV-stabilised, so I used the second and third-thinnest varieties. So far, after nearly a year in place, the material shows no sign of ageing, tearing, or discolouring.

 The vinyl of the outside wall comes down to within about a centimetre of the ground. The gap provides ventilation, which a greenhouse needs. (Some visitors worry that the greenhouse encloses the bathroom window, and fear there will be “no fresh air” for the window. I explain that a greenhouse is a windbreak but not an airlock. In fact the air inside it is always fresh.)

To foil the ubiquitous Australian brushtail possum (a cat-sized leaf-eating marsupial, only distantly related to the carnivorous American opossums)  fruit-protection netting is stretched tautly a centimetre or two above the roof. It does not lie flat on the roof. If it did, the possums would soon learn to run over the netting without tangling their claws, and their sharp claws might cut holes in the roof.

Bernard Davis’s poly-piping and vinyl greenhouse uses the existing house walls for support. Note the white bird-netting stretched above the greenhouse roof (and over part of the house-eaves from which possums might otherwise access the greenhouse roof). Note also the small soft-drink bottle, whose orange top is visible. This is filled with water and suspended from the netting as a weight to hold it taut. Not classy perhaps, but effective.

I have learnt it is possible, in this minimal unheated greenhouse, to grow plants like babaco and pepino (Solanum muricatum)  and passionfruit, which tolerate cold winters but resent frost on their leaves. (The heat coming through the house wall has so far been enough to keep off Canberra’s mild frosts. Minus 6 Celsius was the coldest night last winter, 2013. ) 

 In winter the greenhouse begins to warm up from the very first rays of dawn, which penetrate directly into it through the transparent side walls. By noon the heat-trapping greenhouse  effect is strong. Even in midwinter, when maximum temperatures outside are often only about 10 degrees, babacos continue to ripen and drop fruits (though the main formation and growth of fruits needs to take place during the warmer months.) By the way, Canberra’s inland climate is very sunny, with mean daily sunshine of 7.6 hours/day. It probably gets more sunlight than the coastal warm-temperate city of Sydney to its North, where frost is rare.

In summer the siting is not so ideal. Partly because deciduous trees in the garden are then in leaf, it is not till around 9 a.m. summertime (8 a.m. real time) that the sun’s rays hit the translucent roof and begin to refract sunlight upon the plants below. By 10 a.m. summertime the first direct rays come through the transparent side walls, and from 11 a.m. to 2 pm the greenhouse is hot (and needs to have at least its window and its roll-up door open, if the outside temperature is approaching 30 Celsius). However by 3 pm the sun is starting to leave the greenhouse, and by 4 pm (summertime) the heat-stress has passed, with only the roof and a bit of the Western side wall still getting direct sunlight. 

Here the roof, of translucent polyweave, has been partly rolled up to allow heat to escape during a heatwave in February 2014. Three days of 40 degrees Celsius did not harm the babacos. The side wall of transparent vinyl is almost invisible in the photo.

 It seems that babacos love these protected conditions. By contrast, when grown outside they don’t like hot facings; and in my experience they fail to set fruit if grown in dry and windy places.

 Inside the lean-to greenhouse, our babacos’ trunks tend to slope outwards away from the house-wall and towards the sun.  This, plus the shallow soil and the sheer weight of the fruits (with up to 8 kilos of fruit at a time on each trunk) meant that they needed some support or restraint. So I simply tied them back to an attachment-board on the wall, using old stockings. 

 How high can a Babaco go?
Babacos, once they are a metre high and growing strongly, form a new fruit with each new leaf. Hence there is soon a staggering amount of fruit on a single trunk, with large nearly-ripe fruits lower down and smaller and greener fruits higher up.  Yet this process cannot go on forever. Although Louis Glowinski in his fruit book refers to “the squat babaco” (by contrast with the leaner pawpaw) the trees do eventually get 3-4 metres high. At this height they tend, if grown in the open, to topple over in strong winds. (They are of course best grown in a sheltered valley.)  So the usual practice is to cut down each trunk after it has borne perhaps 20 fruits (sacrificing the many small unfinished fruits at the top) and allow a single lower shoot to replace the lost trunk. (Babacos make side-shoots more readily than pawpaw/papayas do, so there are always shoots in waiting.)  The plant is then out of production for several months until the new trunk is large enough to begin forming fruits.

 Meanwhile the removed top of the trunk is divided into metre or half-metre lengths that are used as giant cuttings.  My practice is to remove all the fruits and the larger leaves from such a cutting, enclose the remaining smaller leaves near the tip in a plastic bag with a few drops of water, and lay the plant on its side in the shade. However I leave the base of the trunk outside the bag and exposed to dry air for at least three or four days till it has begun to callus over. Then I plant it in a large pot (with ordinary potting mix, and use a few empty smaller plastic pots, jammed in beside the trunk, to hold it upright until the roots grow). The cutting stays in a greenhouse or other shaded place, and with the plastic bag over the top for a month or so. Then the bag is removed; and the plant is hardened off for a further month, before it can be moved outside.  Of course in a very humid climate fewer precautions need be taken against the cuttings drying out.

 My original babaco plant has now multiplied into over a dozen, of which about 5 small and 3 large ones are growing in the greenhouse.   (I also have a few for sale.)  While in theory babacos might be allowed to grow to their maximum height inside a sheltered greenhouse, our greenhouse is not quite tall enough to make this possible. So once their leaves start pushing up against the roof, it is time for me to remove the main trunk, and let a side shoot start to replace it.  The aim is to have about half of the plants at any one time in maximum production phase. This means that they currently have a well grown trunk, with ripe and ripening fruits on it. The trunk should have emerged into good light and not yet be too near the roof. 

 I hope others will try this system for cultivating babaco.

Finally, you may be wanting to ask”

“What are babaco fruits like?  Are they similar to papaw/papaya fruits?”

They are similar in size, but their ridged-torpedo shape is distinctive, and their taste is quite different.

As fruits, Babacos are superior to pawpaw/papaya in the following ways:

·        There is no seed-mass to throw away. In fact no seeds at all. Babacos are a seedless hybrid between two Carica species.  They set fruit automatically without pollination. (Since there are no seeds, humans have perpetuated this chance mutation, which may have occurred just once, for many hundreds of years by striking cuttings.)

·        There is no rind to dispose of. The entire fruit is edible, including the skin. It is thus a theoretically perfect fruit like the strawberry, or the Large Oval feijoa.

·        It has a kind of effervescent taste—hence the marketing name “champagne fruit”.

·        Unlike pawpaw/papaya, babacos have a strong and (to me) delicious scent that is an important part of the taste. When mixed into a fruit salad they are one of the most pungent and enriching ingredients.

·        They are also an interesting replacement for figs in fichi e prosciutto

·        The fruit goes on ripening after picking, but quite slowly.

·        It is remarkably tough. If one part is bruised when it falls off the tree, the bruise tends not to spread. Babaco fruits can be transported with less protection than pawpaw.

·        Fruits are edible even when green. The green fruits are sub-acid, and so may be used, cooked, in any recipe where you would use tomato or pimiento.

·        Babacos, like pawpaws, contain the meat-tenderizer papain.  This makes them particularly good as a vegetable to use in curries or with meaty stews. Hence when the trees are producing heavily, babacos can be used both as a fruit and (while still green) as a cooked vegetable.

 Babacos are inferior to papayas/papaws in the following ways:

·        They are not nearly so sweet. This may be a serious problem for people in Western countries who expect even tomato sauce to be full of  sugar. 

·        Whereas almost everyone likes pawpaw/papaya on first tasting it, some people don’t like babaco, or only care for it with lashings of sugar. (One solution is to combine it with ice-cream, for which it has an affinity.)

·        Many people don’t know any recipes for cooking it. Also some, being unfamiliar with babaco, remove the thick edible skin—and then complain that the soft inside part lacks texture.

·        Though there are all kinds of tropical flavours in babaco, the dominant taste is a bit like an effervescent lemon-scented watermelon, which some people find un-exciting.

·        Unlike pawpaw, it is not much improved by adding lime or lemon, since it is already sub-acid. Apple-juice or apple-concentrate is a better idea.

·        Babacos are somewhat watery, especially when over-ripe. Some people, unfamiliar with the fruit, over-ripen it to the point where it has little texture and too much fluid.

·        To counter this, fruit should picked before it is uniformly yellow. If left on the tree till yellow it also tends to drop, and bruise. 

·        Once over-soft it is not suitable for cooking, though it may still make a great smoothy. (When green it is quite firm and chewy, even after being cooked in a stew or curry.)