It is a notion held sacred among Britons who keep vegetable patches and allotments, that "fresh is always best" when it comes to eating.
But despite the belief that their toils are resulting in supremely wholesome and nutritious crops, some home gardeners may be shocked to discover that the fruits of their labour could be no better than week-old bagged salad from the supermarket.
In the first study of its kind, the University of Reading has found that levels of cancer-fighting nutrients in supermarket rocket leaves peak between five and seven days after being processed.
The tests, preformed on commercially grown rocket, found initial levels plummet during the time between picking and washing.
But after a week of being left in the fridge at four degrees, levels had risen threefold, suggesting that as the leaves slowly die, some of their nutritional benefits are actually enhanced.
Sulforaphane, a compound found in rocket, watercress and broccoli which helps give foods "super food" status, has properties which protect against forms of cancer including prostate and gastrointestinal.
High levels of the compound are also the reason behind rocket's peppery flavour.
It forms when glucosinolates present within the plant join with enzymes when it suffers damage, and naturally defends it against being eaten by bugs.
The harvesting process triggers the formation of sulforaphane in rocket, with more "violent" methods, such as mechanical farming, triggering a greater initial fall in levels.
However, the study has established that sulforaphane levels in rocket show "surprising" recovery, peaking at the point around five days to a week after being processed.
It means freshly picked rocket which is just hours old may contain less sulforaphane than leaves which have been sitting in the fridge for days on end.
Although levels of sulforaphane in rocket have been found to increase over time, the presence of some vitamins and other nutrients in the leaves will decline over the same period, affecting their overall health benefits.
Dr Luke Bell, a food chemist from the University of Reading said: “The discovery is really surprising, going against the assumption that nutrients found in rocket will dissipate over the period of time following harvest.
“Our study has shown that the processing actually has a potentially beneficial effect to consumers, and that rocket lovers can have confidence in the health boost a bag of rocket will give them. The biggest boost in these cancer-fighting compounds came seven days after processing, but begin to tail off after that.
“With regular consumption of rocket and sulforaphane, consumers could potentially improve their long-term health and reduce the risk of developing other chronic diseases, such as cardiovascular disease.”
A spokesman for the Leafy Salad Growers Association said: “Rocket is a fantastic healthy and versatile product. The time it takes for packing and distributing the product to retailers ensures the leaves reach the shelves in optimum condition. Rocket is best stored in the fridge and consumers are advised to follow the use by date on the packaging.”