Question: I understand that many diseases overwinter in fallen leaves, and removing these helps “sanitize” the garden. Rotating crops also helps, of course. But can certain diseases (if so, which ones?) overwinter in or on mulch, raised bed wood, fencing, or other surfaces? Our mild climate on the Southern Oregon Coast doesn’t typically get a hard frost, which might affect the viability of some pests and diseases. I generally use redwood or fir chips (and/or needles) or alder chips (smaller pieces from chainsaws or milling) for mulch, depending on availability. Any suggestions would be most welcome. Thanks. -Donna L-C

Answer: Hi Donna,

Great question!
Yes, diseases can overwinter and persist in mulches, potentially transferring to your garden. Since you don’t have a hard winter, it’s more likely these pathogens will survive and spread.

When we say a disease or pest overwinters, we mean it survives through the winter. In regions without hard winters, there’s less natural die-off, which is why warmer climates often experience more severe or varied pest problems compared to areas with harsh winters. That said, pests and infestations still happen in colder climates; it’s just not as widespread or persistent.

We can’t stop using organic matter entirely—it’s too beneficial for the garden. You could sterilize it, but that’s pretty time-consuming. Plus, remember that alongside pathogens, there are beneficial microbes and nematodes in the soil that play essential roles.

Interestingly, studies on cutting boards have shown that wood can be naturally antibacterial. Even after a tree is dead, many hardwoods retain properties that inhibit bacteria, though science hasn’t fully figured out why.
Personally, I don’t worry too much about wood mulch spreading pathogens. The more important factor is ensuring your mulch source is safe. Some mulch companies mix treated woods, telephone poles, or other contaminants into their products, which are fine for decorative landscaping but not safe for food gardens. Be sure to ask the company what materials they chip and whether their mulch is safe for edible gardens or only for non-edible landscaping.

Hope this helps, and thanks for chipping in with such a thoughtful question!