The character of garden vegetables has been altered since the early 1800s due to the technology of commercial production, the tastes of the consumer public, and even the function of the vegetable itself. Some of the variations that distinguish modern varieties from their nineteenth-century parents include insect and disease resistance, a fruit suitable for shipping, compact plants, more consistent harvesting dates, and more cosmetically pleasing fruits.
Most vegetable species are annuals and thus are lost easily if seed collection is neglected. Names have been changed for commercial purposes. Jefferson often listed varieties according to the person from whom he received the seed ("Leitch's pea"), its place of origin ("Tuscan bean"), or else he noted a physical characteristic such as color ("yellow carrot") or season of harvest ("Forward pea"). "Leitch's pea" is not only unavailable from commercial sources today, but there is no description of its qualities in the garden literature of the last two centuries. The collection of Jefferson's 250 vegetable varieties is a complex challenge. In many cases, the varieties planted in the garden today were known in the nineteenth century, and serve as substitutes for the originals grown by Jefferson.
A major influence on Jefferson's gardening practices was Bernard McMahon, a Philadelphia nurseryman and author of The American Gardener's Calendar, the most complete American work on horticulture in the first half of the nineteenth century. McMahon's book provides directions for manuring the garden, interplanting lettuce and radishes, cultivating unusual vegetables such as tomatoes and sea kale, and planting cucumbers in hogsheads. These practices were duplicated carefully in the garden at Monticello. McMahon also sent Jefferson important vegetable varieties such as the Leadman's Dwarf pea, the Egyptian onion, Early York and Sugarloaf cabbage, red celery, and red globe artichoke.
In 1793, Martha Randolph wrote her father from Monticello and complained of insect damage in the garden. Jefferson's response summarized a basic philosophy of gardening:
"We will try this winter to cover our garden with a heavy coating of manure. When earth is rich it bids defiance to droughts, yields in abundance, and of the best quality. I suspect that the insects which have harassed you have been encouraged by the feebleness of your plants; and that has been produced by the lean state of the soil. We will attack them another year with joint efforts."
How much manure Jefferson spread himself is a matter of speculation; however, in his daily attention to details in the garden as evidenced by the "Garden Kalendar" suggests that he was at hand, perhaps directing the work. Jefferson's slave Isaac recalled that "For amusement he [Jefferson] would work sometimes in the garden for half an hour in right good earnest in the cool of the evening."