When and What is Mould Season?


“Mould season” is not exactly something the average person marks on their calendar annually. However, indoor mould does not care if it goes unnoticed during its peak growth periods. 

What Time of Year is Mould Worst? 

Like other natural phenomena, mould experiences its growth cycles. Climate certainly affects mould growth, with some seasons providing better conditions than others. 

Early spring to late summer is considered mould season, with midsummer being the peak time for mould growth. This is especially true for a climate like Ontario, which experiences very humid, hot summers. With summer at its end, now is a great time to check your home for any mould that may have grown this year. The broad availability of damp, moist air checks two of the three boxes required for mould growth, considering the outdoors only. 

This means that mould only requires an organic food source with a cellulose base during mid-summer. Considering that Ontario homes can get damp and humid even with air conditioning, this is terrible news. 

This is because mould can consume construction materials while surviving a comfortably air-conditioned space. Examples of mould food are wallpaper, plaster, wood boards, and human dust or oils. 

In other words, allergy season tends to align with peak mould growth. 


How Each Season Affects Mould Growth

Is mould worse in summer or winter? The reality is that both seasons can be just as bad, considering the conditions for indoor mould growth are so readily available. 


Spring is what ‘activates’ mould. The increasing temperature and constant rain provide an excellent seedbed for mould spores to begin to grow again. Dormant mould spores will activate during this time. 


Mid-summer is the best season for mould growth, as discussed above. Spring run-offs and rainwater dissolve into humid air when the summer heat comes around. Mould thrives off warm, moist air, making it easy to grow wherever a food source is present. 

Fall and Winter

Fall brings cooler weather with it, which slows mould growth in terms of climate. Mould spores require a specific temperature range to thrive. Specifically, mould cannot grow below 40 degrees F, or 4 degrees celsius. This is why mould has a more challenging time to grow in a refrigerated space. Given that winter temperatures are much colder than a fridge, mould spores go into ‘hibernation’ for the winter months. 

Does Mould die in Winter?

No mould does not die during the winter months. occurs due to the stability of indoor climates. Though mould cannot replicate or grow at below-freezing temperatures, spores can enter into a ‘hibernation’ period until conditions change. 

It’s the same reason why mould is able to appear seemingly out of nowhere in almost any location. There are deactivated mould spores all around you, and they only activate when there is enough heat, moisture, and food available. 

Because of indoor heating used during the winter, it is still possible for mould to grow indoors despite the dry winter air. Only
mould remediation can truly kill mould completely. 


What you Can Do to Prevent Indoor Mould Growth Year Round

Though it is only possible to remove mould with remediation, there are plenty of steps to ensure your home is mould-free year-round. The answer to mould prevention is to understand the three requirements of mould growth, how the current season encourages or discourages growth, and how to starve these conditions.

In Summer and Spring

  • Use a dehumidifier to reduce moisture in the air. Moisture levels above 60% are what cause mould growth

  • An air conditioner acts as a dehumidifier, so run that as well

  • Clean up any spills immediately

  • Open a window and allow for some ventilation occasionally. 

In Fall and Winter 

  • Clean any spills immediately 

  • Use proper ventilation in rooms like the shower or kitchen

  • Open a window once in a while to increase ventilation and dry air


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