Usually yes — not because the plant is different, but because of time. Vitamins start breaking down the moment a vegetable is picked, and shop produce is typically days old before you eat it. Spinach kept at fridge temperature (4°C) lost about 29% of its vitamin C within 24 hours of harvest, and up to 94% after seven days.
The plant itself isn't magic — a supermarket spinach leaf and a windowsill spinach leaf start out much the same. What differs is how long ago it was cut. Picking and eating on the same day skips the part where most of the loss happens.
Spinach kept at fridge temperature (4°C) lost about 29% of its vitamin C within 24 hours of harvest, and up to 94% after seven days. In a Penn State study, spinach held at 39°F kept only 53% of its folate and carotenoids after eight days. At 50°F it lost 47% in six days; at 68°F, four days.
Because they were picked before they were ripe. Fruit destined for a long journey is harvested firm and under-ripe so it survives handling, then ripens in transit or storage. Colour catches up; the sugars and aroma compounds that develop on the plant largely don't. A tomato left to ripen on the vine and eaten the same day is a different food, which is why the difference is so obvious the first time you grow one.
No. The crops where freshness matters most — salad leaves, spinach, herbs — are exactly the ones that grow best in a pot on a windowsill or balcony, and they're the fastest to harvest. You can go from picking to plate in under a minute, which is the one thing no supply chain can compete with.
It's the simplest way to control what does and doesn't go on your food — everything on this site uses compost and homemade feeds rather than chemical fertiliser, and physical or plant-based pest control rather than synthetic pesticides. It also tends to mean eating more vegetables, simply because they're there and they need picking.
Freshness matters most for the things that wilt fastest, and those happen to be the quickest to grow in a pot:
Which of these is worth your six weeks depends on your light, your space and the month. The free web version works that out — no download.