Gordon Johnson, Extension Ag Agent, Kent Co.; gcjohn@udel.edu and Tracy Wootten, Extension Horticulture Agent, Sussex Co.
There is a large acreage of vegetables grown using plasticulture in Delaware. The following are some common problems that we often encounter with vegetables grown on plastic mulch.
Fertilizer Damage
There are several ways that fertilizers can injure plants grown on plastic. The most common problem is salt injury. In bed forming and plastic laying operations, caution must be used when using banded applications of fertilizers. Fertilizer banding attachments on plastic layers should be set well to the side of where the plant row will be (>6 inches). Bands too close to where plants are set may lead to salt injury. It is better to distribute the fertilizer over the area to be covered and incorporate it 4-6″ before bedding and laying plastic. Some growers use dilute fertilizer solutions with the plant water. Care must be taken to keep the salt levels low (less than 1% solution is safe – see the specific label recommendations for the fertilizer product being used). Ammonia volatilization and toxicity can be a problem with use of urea or high ammonium containing nitrogen fertilizers under plastic. At least 50% of the nitrogen source should be in nitrate form.
pH Issues
We often see problems with low pH in plastic beds. In cantaloupes, this has led to manganese toxicity in the past. Make sure that the soil pH is corrected by liming well ahead of laying plastic. Remember that pH will drop some with the use of ammonium nitrogen fertilizer sources under the plastic and soils with a marginal pH (around 5.8 for example) can drop down to levels that will affect plant growth (5.3 or lower).
Transplants With Small Root Balls
A common problem we see is the use of plants with inadequate root balls that are then susceptible to wilting and desiccation with high winds, especially during cold snaps.
Planting Depth
Poor plant performance or plant loss can result from setting plants too deep, especially with smaller plants where the soil covers growing points and from setting plants too shallow where part of the root ball is exposed, causing plant desiccation.
Excessive Transplant Shock
Plant loss often occurs by allowing plants to dry out too much prior to setting (sitting long periods in the wind on wagons for example), by allowing plant trays to heat up too much in the sun, or by planting into black plastic on very hot sunny days.
Heat Necrosis
As black plastic heats up, the temperature right around the plant hole can get so hot that it will kill the plant tissue nearby, right at the soil line, causing the plant to die. This occurs most often where plant stems touch the plastic mulch and with plants that have small stem diameters.
Improper and/or Malfunctioning Drip Irrigation
Vegetables grown on plastic mulch require more attention to providing proper irrigation. Common problems include under-watering (keeping beds too dry), over-watering (leaching out plant nutrients), plugged drip emitters causing dry spots, holes in drip tape causing wet areas at the leak and a under-watered bed beyond the leak, and pressure losses resulting in drip tape not operating properly and giving poor water distribution.