When the Pilgrims came to Plymouth colony, they weren't able to rely on regular supply ships from England. They had to become self-sufficient as soon as possible. That meant growing much of the food they needed. They faced three main problems: experience, temperature and soil.
The Pilgrims came mainly from around London or from Holland, where they had lived in exile for 10 years. Being urban dwellers, most of them had little to no experience with farming. Written farming resources in the 1600's were almost non-existent, and in any case, few of the Pilgrims could read.
Cold hardiness zones in England range from the equivalent of US zone 7a to 10a, with the zone near London being 8b (15 to 20 degrees F); zones in Holland range from 8a to 9b. The cold hardiness zone around the Plymouth area is 6b (-5 to 0), which is significantly colder. The growing season was shorter, probably unexpectedly shorter.
The shallow, sandy, stony soil was also vastly different from the deep, nutrient-rich loams of southern England. It did not retain water nearly as well, so many of the crops the Pilgrims brought with them grew poorly or died due to lack of water. For a detailed examination of the soil challenges the Pilgrims faced, read the article: "WHAT TYPE OF FARMING CHALLENGES DID THE PILGRIMS FACE?"
Had it not been for the techniques (like the "three sisters," growing corn squash and beans together), the crops (Corn and squash were unknown in the Old World, and the bean varieties common there did not do as well here.) and the knowledge of local wildlife (Turkeys are native to the Americas.) shared by their Wampanoag neighbors, the Pilgrims would very likely have starved their first winter in the new world, and our concept of the Thanksgiving feast, if we even had one, would be vastly different.