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Heat mat question #927012

Asked March 29, 2026, 2:12 PM EDT

When should I turn off a heat mat on when starting seeds?

Hennepin County Minnesota

Expert Response

Thank you for writing.
The question is pretty non-specific. 
Are the plants in a heated green house or out doors?
What kind of lighting do they have natural (how many hours a day?), LED, incandescent?
What kind of plants are they?
Generally, for April May plantings, they do not need a heating pad and can do well under a cold frame.   
&t=6sMy cold frame is simply clear plastic on top of my veg garden boxes

https://www.bootstrapfarmer.com/blogs/how-to-grow-seedlings/how-to-use-heat-mats-for-starting-seeds?srsltid=AfmBOoqosVlkYB_DDMNK0RS4YoxWAuDscXRToDa-Ye8nWmSrESuKD8Mt
Steve Miles, Master Gardener, Hennepin County MN (I am a human being, not a bot nor AI). I am an avid gardener but am not a degreed or certified horticulturist or arborist. I am happy to receive follow-up questions if my responses are incomplete or unclear.  This is the University of MN' garden soil testing service.  https://soiltest.cfans.umn.edu/testing-services/lawn-garden This is guidance on how to find a tree expert.  https://extension.umn.edu/planting-and-growing-guides/how-hire-tree-care-professional The Minnesota Landscape Arboretum is a wonderful place to see and learn about plants that grow in Minnesota. https://mngardens.horticulture.umn.edu/minnesota-landscape-arboretum Replied March 29, 2026, 4:27 PM EDT
I'm starting butterfly weed indoors, with grow lights. They've started germinating a couple of days ago and now most are up. I'm not sure if they still need the heat or not.

On Sun, Mar 29, 2026, 3:27 PM Ask Extension <<personal data hidden>> wrote:
The Question Asker Replied March 29, 2026, 4:50 PM EDT
You can stop the heat pad.
This plant requires cold stressing. 
It may germinate but cold stressing is required to help it meet its full height. 
https://yardandgarden.extension.iastate.edu/faq/how-do-i-germinate-butterfly-weed-seeds
Steve Miles, Master Gardener, Hennepin County MN (I am a human being, not a bot nor AI). I am an avid gardener but am not a degreed or certified horticulturist or arborist. I am happy to receive follow-up questions if my responses are incomplete or unclear.  This is the University of MN' garden soil testing service.  https://soiltest.cfans.umn.edu/testing-services/lawn-garden This is guidance on how to find a tree expert.  https://extension.umn.edu/planting-and-growing-guides/how-hire-tree-care-professional The Minnesota Landscape Arboretum is a wonderful place to see and learn about plants that grow in Minnesota. https://mngardens.horticulture.umn.edu/minnesota-landscape-arboretum Replied March 30, 2026, 9:33 AM EDT
I did cold stratify the seeds first and 90% are germinated.

On Mon, Mar 30, 2026, 8:33 AM Ask Extension <<personal data hidden>> wrote:
The Question Asker Replied March 30, 2026, 10:10 AM EDT
Thats is good to know. 
Take the mat away. You can plant any time.
Steve Miles, Master Gardener, Hennepin County MN (I am a human being, not a bot nor AI). I am an avid gardener but am not a degreed or certified horticulturist or arborist. I am happy to receive follow-up questions if my responses are incomplete or unclear.  This is the University of MN' garden soil testing service.  https://soiltest.cfans.umn.edu/testing-services/lawn-garden This is guidance on how to find a tree expert.  https://extension.umn.edu/planting-and-growing-guides/how-hire-tree-care-professional The Minnesota Landscape Arboretum is a wonderful place to see and learn about plants that grow in Minnesota. https://mngardens.horticulture.umn.edu/minnesota-landscape-arboretum Replied March 30, 2026, 7:16 PM EDT

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