Cucurbits

• Pumpkins, gourds and winter squashes are regarded as cucurbits, a family of plants with vining stems that produce yellow flowers followed by firm-skinned fruits. Botanically, there is no one species of plant that is a pumpkin. In fact, pumpkins are an American colloquial name assigned to any gourd or squash that develops a distinctive shape and orange color. Three cucurbit plant species can yield pumpkins: Cucurbita moschata, C. maxima and C. pepo. None is harmed by the toxin created by black walnut trees in garden soil.
  
  

Juglone

• While pumpkin plants typically don't demonstrate adverse affects from growing in proximity to a black walnut tree, some cultivars or garden site conditions could be problematic. The closer a pumpkin plant grows to the trunk of a black walnut tree, the higher the juglone concentrations in the soil. An unhealthy, weak-growing pumpkin plant can be sensitive to juglone in high enough concentrations, although that isn't the rule. Garden soils rich in compost and manure support lots of microorganisms that diminish juglone and benefit the growth of pumpkin plants.
  
  

Soil Conditions

• Examine the soil around the black walnut tree to first determine if good pumpkin growing conditions exist. If the tree is large and has a thick matrix of roots, cultivating the soil to plant pumpkin seeds may not be so easy or worthwhile. Dig into the soil where you want to grow pumpkins. Pumpkins grow best in a crumbly soil that is rich in organic matter and evenly moist. If the soil near the black walnut tree is rock hard or littered with roots, it's not an ideal garden bed location. Pumpkin roots and tree roots absorb the same nutrients and water in the shared soil area.
  
  

The Need for Sunlight

• The only major caveat surrounding planting pumpkins near walnut trees is the issue of sunlight. Pumpkins need warm soil, warm temperatures and at least 10 hours of direct sunlight daily to grow well. Planting too close to a black walnut tree may limit the amount of sunlight and warmth. Certainly do not plant pumpkins on the north side of a tree, as too much shade exists. Situate pumpkins on the south side so there is no issue of lack of sunlight for robust leaves and ample production of flowers.