Gordon Johnson, Extension Vegetable & Fruit Specialist; gcjohn@udel.edu
I have been asked several times in the last two weeks about double cropping vegetables. The following are some considerations in double cropping.
Crop residue management is critical in order to get a good seed bed for the double crop vegetable. Make efforts to spread and incorporate residue evenly. Heavy areas of incorporated straw or vine will lead to crop variability. Incorporation of high carbon materials such as small grain straw can lead temporary nitrogen deficiencies. Therefore, extra nitrogen fertilizer will be needed to speed decomposition of heavy straw residue. In contrast, green materials such as pea vines will not cause nitrogen tie-up and will rapidly decompose. It is advised to allow some time (minimum 5-7 days) for residue decomposition before planting the next crop.
Allelopathic responses (toxic reactions) in the double crop planting have been found in certain cases when planting has occurred immediately after incorporation of residues. One example has been with pea vines. Fresh pea residues have been shown to release pisatin, a chemical which can inhibit the germination and seedling growth of some seeds. Allowing residue to dry before incorporation, moldboard plowing fresh residue so it is out of the germination area, or waiting for a period of time to allow incorporated residue to decompose and the pisatin to break down before replanting (7 or more days) would reduce the risk of this allelopathic reaction affecting following double-cropped vegetables.
Pay close attention to herbicide plant back restrictions for vegetable double crops. Low rates (0.5-0.75 lbs) of atrazine are often used in sweet corn and this normally does not affect subsequent plantings. However, higher rates can damage the double crop planting. Mesotrione (Callisto, component of Lexar and Lumax) which is used in sweet corn has significant replant restrictions to many vegetables as do topramezone (Impact) and tembotrione (Laudis), also labeled for sweet corn. Command, Reflex, and Pursuit are examples of other common herbicides with significant plant back restrictions. Check the Delaware Commercial Vegetable Production Recommendations book and the specific herbicide labels for appropriate waiting periods and crops rotational restrictions.