Opinion is divided about what plants produce the largest leaves, flowers and fruits in the plant kingdom. A quick search of the internet reveals lots of different answers but the following seems to be a realistic summary. Pumpkins for fruit,  Rafflesia or Amorphophallus for flower and Raphia for leaf.

Supersized Fruits
Those keen gardeners who grow giant pumpkins will be quick to say the pumpkin is the world’s largest fruit as strictly speaking a pumpkin is classed as a fruit as it contains seeds. The cultivar ‘Atlantic Giant’ can produce fruit up to 450kg in weight. That’s a lot of pumpkin soup! I believe a specimen of the world’s largest watermelon cultivar ‘Carolina Cross’ weighed in at around 120kg and the tropical jackfruit Artocarpus heterophyllus can weigh up to 36kg each. In the grounds of the Darwin Botanic Gardens I photographed the cannonball tree Couroupita guianensis from South America. It has palm-sized orange flowers followed by spherical fruits the size of a cannonball. The sausage tree Kigelia pinnata comes from South Africa and has fruits over 50cm in length. The coco-de-mer or sea coconut Lodoicea maldivica is a type of coconut 40-50cm in diameter and produces the largest seed in the plant world. From Madagascar comes the royal poinciana Delonix regia which has seed pods up to 60cm in length – they must pose a challenge (with regard to disposal of the pods) for gardeners in the tropics!

Supersized Flowers
The world’s largest flower is a toss up between Rafflesia arnoldii and the titan arum Amorphophallus titanum. The former is a parasitic plant found only in Indonesian rainforests and can measure up to 1m in width. It does not produce leaves – the single flower just emerges from the litter on the forest floor. The latter is from Sumatra but has been grown and flowered in several botanic gardens around the world and its unbranched inflorescence can reach 3.5m in height and about 1m wide. But don’t get too enthusiastic – they are very exacting in their conditions required for growth (in fact I don’t think anyone has grown the rafflesia) and they both stink when they flower!

Supersized Leaves
The largest leaf in the world apparently belongs to the raffia palm Raphia regalis a tropical African palm with pinnate leaves up to 24m in length. But you could also argue that the leaf of the amorphophallus (6m long by 4.5m wide) is pretty big, as is the leaf of the biggest of the elephant ears Alocasia robusta which can be 3.5m in length.

What Might Grow in your Garden?

Leaves
For large leaves any of the elephant ears could be chosen and there are species able to grow from cold climates through to tropical climates. They certainly add that tropical touch to any garden. Bananas Musa sp. are great for producing huge leaves as is the giant bird-of-paradise Strelitzia nicolai and the Chilean rhubarb Gunnera manicata if you don’t mind the prickles. Tree ferns such as Cyathea australis or Dicksonia antarctica produce huge fronds up to a couple of metres in length or more. The largest leaves in my garden belong to the ox tongue lily Haemanthus coccineus. I once saw leaves on a Chatham Island forget-me-not Myosotidium hortensia as large as a platter – but that was on Chatham Island – I guess they should grow pretty well in their native habitat! That reminds me that the leaves and flowers of the megaherbs* are all pretty big but unless you live on a subantarctic island, you can forget about growing them.

Flowers
The largest flower I have grown in my garden would have to be the herbaceous hibiscus ‘Fireball’. Although the plant only grew to about 1m in height, each flower was the size of a dinner plate – about 30cm wide! The tallest flower I have grown was Echium ‘Cobalt Tower’ and it towered over our heads – 3m from the base of the plant to the tip of the flower spike. Other tall flowers in my garden are the flower spike of giant fennel Ferula communis and the NZ flax Phormium tenax. Agapanthus flower heads can be the size of a basketball and make a great sight flowering en masse as can some of the enormous dahlias and chrysanthemums. Soon my bright red Aloe arborescens X ferox will bloom and its flowers measure well over 30cm in length; often with 6 held in each branched inflorescence. It is quite a sight!

Fruit
One large fruit that grew at my childhood home belonged to the fruit salad plant Monstera deliciosa. Once the roots had grown out of the pot and into a neighbouring garden bed, the plant took off and regularly produced huge flowers followed by huge fruits the size of a rolling pin that we took inside, ripened in a brown paper bag with a banana and then ate! Pomelos produce huge fruits up to 30cm in diameter (no wonder it’s called Citrus maxima). Don’t forget that the fruit (or more correctly seed cone) of our native bunya pine Araucaria bidwillii can get up to 30cm in length and weigh several kilos. Never park your car beneath one or walk under their canopy during their late summer/autumn fruiting season!

*Megaherbs include Pleurophyllum, Bulbinella, Stilbocarpa and Anisotome. You can get some idea of one subantarctic island and its flora – Macquarie Island – by visiting the Subantarctic House in the Royal Tasmanian Botanic Gardens. 

http://en.wikipedia.org/wiki/Megaherbs