Tulips grow from bulbs and can be propagated through both seeds and buds. Seeds from a tulip will form a flowering bulb after 7–12 years. When a bulb grows into the flower, the original bulb will disappear, but a clone bulb forms in its place, as do several buds. Properly cultivated, these buds will become flowering bulbs of their own, usually after a couple of years. The tulip breaking virus spreads only through buds, not seeds, and propagation is greatly slowed down by the virus. Cultivating the varieties that were most appealing at the time therefore takes years. In the Northern Hemisphere, tulips bloom in April and May for about one week. During the plant's dormant phase from June to September, bulbs can be uprooted and moved about, so actual purchases (in the spot market) occurred during these months. During the rest of the year, "florists", or tulip traders, signed contracts before a notary to buy tulips at the end of the season (effectively futures contracts). Thus the Dutch, who developed many of the techniques of modern finance, created a market for tulip bulbs, which were durable goods. Short selling was banned by an edict of 1610, which was reiterated or strengthened in 1621 and 1630, and again in 1636. Short sellers were not prosecuted under these edicts, but futures contracts were deemed unenforceable, so traders could repudiate deals if faced with a loss.