Summer in the Southeast brings long stretches of heat, humidity, and sudden rain; conditions that can impact the appearance of wood pallets, crates, and industrial packaging. At McVantage Packaging, we get a lot of questions this time of year about dark spots, discoloration, or “mold‑looking” areas on wood products that have been stored outdoors or in warm, stagnant air.
The truth is: summer weather can change the way wood looks, but those changes rarely affect performance. And just as importantly, not all stains are mold.
In fact, the National Wooden Pallet & Container Association notes that many dark or unusual discolorations are not biological at all, and “there are many types of naturally occurring, non‑biological defects that may look scary, but are not caused by microorganisms.”
Understanding what’s normal, and what’s not, can save your operation time, money, and unnecessary worry.
How Summer Heat Affects Wood
1. Sun Exposure (Weathering)
Direct sunlight causes wood to darken over time, similar to a suntan. Sun exposure causes a chemical change in the tannins of the wood, which is why it can get a “weathered” look. However this is nothing to worry about.
Weathering does not weaken the pallet nor indicate mold, and in many cases it is done on purpose for cosmetic reasons.
If your pallets sit outside, even for a few days,you may see graying, darkening, or uneven color patches. This is normal.
How Humidity & Moisture Influence Wood
Summer humidity is the biggest driver of visible changes in wood packaging. Wood is a natural material that absorbs and releases moisture from the air. When humidity rises, moisture content rises too—and that can lead to:
Bluestain (Often Mistaken for Mold)
Bluestain (or blue stain) fungi thrive when wood moisture exceeds 19%. Bluestain is not mold and it has no effect on the performance and strength of lumber.
Bluestain can appear blue or bluish‑black or gray, and even red, yellow, orange, pink or purple. People often seek bluestain pine for its color.
The key test: if it’s mold it will smear or brush off. Bluestain penetrates the wood and cannot be removed. Bluestain is harmless and safe to handle.
Iron Stain
Humidity + metal contact = black or dark streaks.
Iron stain occurs when extractives naturally occurring in wood react with iron‑containing materials, such as metal fasteners and sawblades.
This stain often appears after rain or condensation and does not affect the strength of the wood.
Enzymatic & Mineral Discolorations
Warm weather accelerates natural chemical reactions inside the wood, leading to:
Gray or brown tones (enzymatic)
Dark streaks, green patches, or purple‑black areas (mineral)
These are naturally occurring and not preventable, nor do they affect performance.
Why Summer Makes These Issues More Noticeable
Heat, humidity and slower air movement, all of which describe summers in the southeast and middle south, are ideal conditions for discoloration.
Summer conditions can increase moisture content, slow drying time, encourage surface fungi (including harmless ones), intensify color changes from sun exposure and accelerate reactions that cause iron stain. This is why pallets stored outdoors in July look different from pallets stored indoors in January, even if they’re the same species, same mill, same production run.
How to Reduce Weather‑Related Discoloration
Based on the brochure’s recommendations and McVantage’s best practices:
1. Keep Wood Dry
Bluestain thrives above 19% moisture. Use tarps or wrap, but ensure ventilation holes are present. Keeping it dry and in low‑humidity conditions will prevent it from growing.
2. Improve Airflow
Avoid stacking pallets in tight, unventilated spaces. Warm, stagnant air is the perfect environment for discoloration.
3. Elevate Your Inventory
A 6–8 inch elevation prevents moisture wicking from the ground. This will keep your products dry.
4. Keep Vegetation Clear
Surface bluestain can be transmitted by insects. Clearing brush and debris reduces risk.
5. Limit Metal Contact
Store pallets away from rusty metal, old nails, or steel dust to prevent iron stain.
The Bottom Line for Your Operation
Summer weather can change the appearance of wood, but it rarely affects the performance of your pallets or crates.
Most discolorations are cosmetic and naturally occurring. They do not indicate mold nor weaken the wood, and they do not require replacement.
At McVantage Packaging, we help customers understand what’s normal, what’s preventable, and how to store wood packaging to minimize weather‑related changes. Contact us if you’d like more information, or a quote on wood pallets and containers.