Pitaya fruiting for you?
At RFC Meetings, members have often reported difficulties getting these plants to fruit,
so I hope the following points might help you coax them into producing for you.
Pitayas (Selenicereus
spp) are climbing understory members of the cactus family, endemic to
the Neotropics where the climate is humid and wet
tropical/sub-tropical, without frosts, high temperatures or low
humidity. Growing them in the Perth region presents a challenge, but
one that can be overcome with appropriate care. The Israelis have been
leaders in pomological research on the species for decades, and they've
shown it's possible to produce very high yields in arid desert
conditions more unfavourable than ours.
Plant Selection
The three main commercial species worldwide are S. undatus (white flesh, red skin, WR), S. monacanthus (red flesh, red skin, RR) and S. megalanthus
(white flesh, yellow skin, WY); the first two are much larger than the
last and probably the ones you’ve seen in retail outlets and are most
familiar with. All three have varying degrees of self-incompatibility,
and all are capable of cross pollinating and fertilising one another.
As a result, some plants sold in retail outlets are actually hybrids of
varying degrees. On average with the three, WY is the most
self-fertile, WR less and RR much less. The large flowers open for one
night only, so natural pollinators are moths and other nocturnal
animals. However flowers can open 1-2 hours before sunset and may
remain open for similar times after sunrise (longer if a cool day), so
if conditions are suitable for bees to forage, they can also
contribute. This contribution can be significant with WY as the stigma
and anthers are close together and compatible with bee size and
behaviour, but is less so with WR and RR as these organs can be cms
apart. In some countries and regions, suitable pollinators are not
present in sufficient numbers and hand-pollination is always necessary
to achieve meaningful yields.
There's a lot of heterogeneity in Selenicereus,
so different clones of the same species (they’re mainly propagated
asexually by cuttings) can differ enough genetically to achieve
adequate fertilisation when crossed. The probability of this being
achieved is about 10%. Crosses between two of the three species ensure
adequate genetic difference, fertilization and good fruit set provided
sufficient pollen transfer is achieved either naturally or by hand. The
resulting fruit are generally larger than crosses within clones for WR
and RR, but usually smaller and less satisfactory when crossed with WY;
WR and RR are both diploid and WY is tetraploid. The problem in
planting several vines of only WR or RR is the uncertainty whether
cross-pollination will be effective during the time you put into
growing them, with failure being likely where plants all come from the
same source. Deliberately planting a mix of both WR and RR overcomes
this, and a further benefit is many F1 hybrids are sweeter than the
parents.
If you’re only going to plant
one vine, WY will give you the best odds of getting fruit. They’re
sweeter than the other two but much smaller, and many people are put
off by the sharp spines. However when fruit are mature these can be
easily brushed off before picking. If you only grow a single WR or RR
plant you could be lucky with it being relatively self-fertile, but the
odds are against you. Even if you plant several WR or RR you may
struggle to get many fruit.
Young Plant Establishment
You
should select a place in your garden where vines will not be exposed to
full sunlight, heat and dry conditions during our warmer months. Ideal
mean temperatures are 20-30°C. If this is not addressed, the plant
switches activity away from reproduction to survival mode and yield
declines several-fold; stem sections will bleach and rot when
conditions are severe. If you don’t have a suitable place that
naturally provides some shade and a suitable micro-climate, then you
should consider using shade cloth with 30-40% radiation block.
In
commercial orchards, there are two main support methods for the vines
to climb on – a sturdy post with a horizontal section at the top, and a
trellis system both about 1.5-2m high. Two or three plants are placed
around each post and trained to grow straight to the top by removing
lateral growth, with further growth and branching then allowed to hang
down from the horizontal support. With a trellis, they can be placed
2-3m apart along the trellis, with similar training to the top before
becoming pendant. The horizontal support should be of sufficient gauge
(ie not a wire) to avoid cutting into the delicate stems that hang
downwards. Supports need to be strong enough to carry loads that may be
greater than 50kg/plant when mature (I learnt this the hard way). If
well managed, pitayas grow rapidly and can start flowering/fruiting in
their second year. Like all fruit trees, non-vertical stems have
greater reproductive potential than vertical. So it's the pendant stems
where you’ll mainly get your fruit, and you need to always manage your
plants to generate these. Leaves in pitaya are reduced to spines, small
with WR and RR and longer with WY.
Management
In
their native environment, rainfall can be more than 2000mm pa. However
they cope quite well with drought conditions by drawing on water stored
in their succulent stems, and they usually won't die unless it's for an
unduly long period as they're very efficient users of water. But like
many other hardy plants (eg olives), if you want good fruit yields you
have to provide fertile conditions and adequate water, plus in this
case good drainage. A little often enough to keep the topsoil moist is
better than the occasional heavy soaking as they have a shallow root
system and are unable to take up water from deeper levels. Mulches and
good soil organic and clay content will help achieve this relatively
constant moisture level. Like water, fertilisers should be little and
often. Israeli studies have shown NPK ratios of 11, 2.5, 28 are best,
with approximately twice the application rate during flowering and
fruiting compared to vegetative periods. As plants are quite sensitive
to salinity, do a check if using bore water.
Flowers and fruit
are borne on new growth, so as with other fruit species in this
situation, pruning after harvest each year will encourage fruitful new
growth for the following season. Learn to recognize the different shape
of vegetative and reproductive buds to see whether you're nudging
plants in the desired fruitful direction - vegetative buds are
slim/pointed and reproductive plump/rounded. Under ideal conditions you
can obtain one or other of these at each node ie where the
spines/areoles are. By planting both WR and RR together, perhaps also
with some genetic diversity within each species, you may get some fruit
without having to hand pollinate. With all three species, fruit size is
governed by how effectively each flower is pollinated/fertilised - the
greater the number of seeds formed (which can be in the thousands) the
bigger the fruit. Larger fruit are produced with hand pollination as
fertilization is more complete given the less-than-efficient vectors we
have. If you don't have flower overlap at certain times you can store
pollen in a sealed bag without loss of viability at 4°C for a few days
and longer at -18°C. Flowers that have not been successfully fertilized
reveal themselves quickly, with the basal scales next to the stem
turning yellow and the whole flower falling off within 4-6 days of
anthesis. The good ones remain green and the enclosed region closest to
the stem will expand and grow rapidly. In their native environment,
anthesis to harvest may be only 35 days. See if yours get near this
with good management. Wait until fruit have developed full colour
before picking as they’re a non-climacteric fruit and will not ripen
further afterwards.
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