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.
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.
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.
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.