Weather Conditions

Planting conditions are most favorable when the:

• Relative humidity is above 50 percent.
• Wind speed is below 10 miles per hour.
• Air temperature is between 35 and 50 degrees Fahrenheit.
• Soil temperature 6 to 8 inches below the surface is higher than 40 degrees.

However, limiting planting to the most favorable conditions could result in a lengthy planting period during which soil moisture could become too low for planting. It is necessary to plant when the planting conditions are not completely optimal. You should recognize the environmental conditions that are most stressful to trees during planting and soon afterward. You may need to shut down planting for the day or a few days to prevent undue tree stress, and in some cases you may have to do so for the safety of the tree planters. There are no definitive answers determining exactly when to shut down one operation or when to move to another unit, but you should use local guidelines and be aware of conditions that reduce tree survival. Seek the assistance of other specialists as needed when determining planting thresholds and when evaluating environmental conditions. Also consider these hazards:

• Snow—Planting spots are difficult to select when the ground is covered with snow. If snow is mixed with soil in the planting hole, air pockets may be left when the snow melts. Air pockets will cause the roots to dry out and kill the seedling. Snow on steep slopes can make planting hazardous.
  
  
• Frost—Planting holes cannot be opened or closed properly when frost is more than ½ inch deep in the ground. When the soil contains frost, winds can desiccate seedlings rapidly because moisture is locked up in frost.
  
  
• Freezing weather—Tree roots and the root plugs of containerized seedlings become brittle when exposed to freezing conditions. Brittle roots are easily damaged. The tops of trees and needles are also subject to damage.
  
  
• Dry soil—Trees cannot be properly firmed in the planting hole or planted deep enough when the soil is too dry and has the consistency of flour or hard clay. Delay planting until rain allows the soil to become workable, especially if there is a chance of rain or snow.
  
  
• Wet soil—Soil closing the planting hole cannot be properly firmed when the soil becomes too wet. Soil may need to be allowed to drain or dry before planting.
  
  
• Winds—Sustained winds, especially when accompanied by low humidity and high temperatures, cause high moisture stress. Planters will have to take especially good care of trees. Consider planting during the early morning and after the heat of the day. In burned areas, recognize the increased risk of falling snags during periods of high wind. Do not allow planting to continue during such periods. Determine maximum allowable windspeeds or temperatures before work begins and evaluate conditions throughout the planting day.
  
  
• High temperatures—More than a couple of hours of high temperatures can cause newly planted trees to experience moisture stress. Consider planting only early and late in the day. (top)

Measures to Mitigate for Weather Conditions

When conditions are not suitable for tree planting in one portion of a unit, consider moving to another portion of the unit or to another unit if conditions are more suitable elsewhere. Sometimes weather conditions will change based on topography or local terrain. Consider altering the planting day when conditions are more suitable early or late in the day. You may want to shut down for a few hours if the conditions are expected to improve later. Finally, when shutting down the planting operation, remember to provide the appropriate contract adjustments. If the contract has weather shutdowns built into the planting period, no contract time adjustments are needed. Know your contract before discussing shutdowns or altered work schedules with the contractor.

Hobbs, Stephen D. and others (ed.). Reforestation Practices in Southwestern Oregon and Northern California. Corvallis, OR: Oregon State University, Forest Research Laboratory, p. 331.