Opuntia robusta is the prickly pear cactus with the largest pads and fruits. The pads are modified stems, not leaves; botanically they are cladodes. They can get nearly a meter across in habitat. It is marginal in Phoenix; our summers are hotter, and winters cooler than it likes. Nevertheless it flowers and fruits each year. Though pads here reach less than half their potential, it is easy to recognize. It is only rarely grown in California, and Texas; it isn't adapted for much winter rain, and tolerates only light frost.
Here you see my plant. I am 5' 11"/180cm tall. The fruits on the third pad from the left center are at my eye level.
It is esteemed in Mexico as among the best tasting prickly pear fruits, called tunas (singular tuna.) Flowers are yellow. In habitat fruits on different plants range in color from green through yellow, orange, red and deep purple. In the US there seems to be only one cutting-propagated clone, with deep red fruits. In Mexico it grows as a large tree, with a stout woody trunk. In common with many other tree-habit cacti, tip cuttings do not form a trunk. My robusta falls apart whenever it gets more than 4-5 ranks of pads high, because only a sturdy trunk could support that weight. More cuttings to distribute!
Almost all cacti have the seed-bearing ovary below the flower petals. This is called having an inferior ovary. In all the prickly pear cacti and relatives, as well as most other cacti, the true fruit is sunken into a modified stem that bears spines. Sometimes the spines fall off when fruits are ripe, as in the North American Echinocereus species, called hedgehog cacti, whose fruits taste like strawberries. Most tunas need to have the spines cut off before eating. They usually have tasteless stem segments around the yummy interior.
The spine clusters, called areoles, are modified branches. Spines are modified leaves. Many prickly pear relatives have fruits that will root and grow new plants. Roots and shoots form from meristems in the areoles.
In prickly pears, hedgehogs, Cereus and dragonfruit, the juicy sweet pulp inside consists of the swollen filaments that carry water and nutrients to developing seeds from to the wall of the fruit. Cacti are unique in this.
I've never tasted my Opuntia robusta fruits before. This year an intensive rodent trapping program let me have some. The sweet inner pulp is very good. It is loaded with rock-hard seeds. I hope to sprout some so I can have some tree-form plants. Maybe there will be variability of fruit color, too. I will also send them to the seed depot of the Cactus and Succulent Society of America.
But what surprised me about the robusta fruit was the wall, the modified stem. In most tunas this is juicy but tasteless. The robusta fruit wall tastes exactly like an almost-ripe paste tomato.
Opuntia robusta 2020
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