Germination of seeds is one of life’s most beautiful phenomena. You take this dead, rock looking thing, put it in some soil, water it, and a beautiful, living, green plant begins to grow. This was always a fascination for me as a kid, and even now as a well-seasoned adult, it is still like watching a bit of magic. Seeds truly are one of nature’s evolutionary breakthroughs. When the plant is near the end of its life cycle (annuals), or is approaching a harsh season (perennials), they go to seed and thus carry on the species by giving rise to the next generation. The seeds then just sit there dormant, waiting for environmental conditions to improve, biding their time patiently. 

Seemingly lifeless tomato seeds waiting for environmental conditions to be right so they can germinate. Photo by mrl2021.

When the time is right, and all the abiotic (non-living) parameters line up correctly, the seeds will germinate. Seeds are a bit like Goldilocks and the porridge in that respect. Too hot or too cold and it is a no go. However, when seeds find themselves in the correct temperature range with the right amount of moisture, they spring into action. It is important to note that there is a range at which seeds can germinate. The trade off, though, is that it generally takes longer to germinate at cooler temperatures. That being said, if we increase the temperature, germination will be sped up considerably. There is an upper end to this range, however, and if it gets too warm they will not germinate either. 

Note that each plant has its own optimal germination temperatures. It truly is not one size fits all, so it behooves the gardener to research the proper germination temperatures for the vegetables you are trying to grow. The nice part is that we can manipulate the process to occur much faster than it would have normally. Many vegetables take seven to fourteen days to germinate, assuming you are using room temperature. Any colder than that will either lengthen the germination time, or inhibit germination all together. 

The best way to speed up the germination process is by adding bottom heat. This is accomplished by using a heat mat. These can be really pricey, but once purchased can be reused year after year. Unfortunately that is not all you will need. These heat mats need to be controlled by a thermostat (control box). Do not attempt to use them without one as it will ensure disaster.  These are many times just as pricey as the heat mat itself. There are a number of different heat mat companies out there. There are some differences so do your research and find the one that matches what you are trying to accomplish. Heat mats come in many different sizes to accommodate the needs of the grower. There are mats suited to one standard 10×20 plant tray, or mats that can hold up to ten of those plant trays. Of course there are many sizes in between those extremes, so you should be able to find one that can fit your space. If you are trying to decide between two sizes, I always recommend going bigger if you can as with any hobby or endeavor humans generally like to expand.  

A heat mat. Photo by mrl2021

You have to choose your seed starting location carefully. First of all, you will need access to an electrical outlet. Extension cords may be okay to use, but make sure they are sized to handle the electrical needs of your mats. The more mats you have, the more amps you will be pulling. Read the information on the electrical cord packaging to determine the proper gauge and buy the shortest cord possible. I always go with a bigger gauge just to be safe. You will plug the mat or mats into the thermostat, and the thermostat into the wall outlet. The thermostat will have some type of mechanism which allows you to set the temperature so read the instructions on how to adjust it and set to the proper temperature for your plants. Don’t forget to change the temperature once you switch crops, if needed. These seed mats work great, but they do have their limitations.  They generally cannot raise the temperature more than twenty degrees above ambient. So while the basement may seem like a great place to start your seeds, it may end up being a little too cool if unheated in the late winter or early spring. The heat mats should be left on for twenty four hours a day until the seedlings are up and well on their way.    

A thermostat used to control the temperature of the heat mats. They will not function properly without one. Photo by mrl2021

The thermostat will have a probe on it to sense temperature. This can only be inserted into one tray, so it is assumed all the trays that fit on that mat will be the same temperature. You want to make sure that the probe has good contact with the soil, and is neither too deep nor too shallow.  One word of caution – the seed starting medium must be kept moist for two reasons. First, if the soil dries out then the seeds won’t germinate. Second, if the soil gets too dry, the probe will not be able to sense the temperature properly and your thermostat will not function properly. This could end up cooking your seeds or not working at all. At the very least, it is best to check the moisture levels twice a day, but three times is even better. I like to water my plant trays with a spray bottle. You have to be very careful not to disturb the soil too much as this may dislodge some sprouting seeds. 

The probe attached to the thermostat senses the heat in the seed trays. Make sure to keep the growing medium moist to ensure proper function. Photo by mrl2021

To help prevent the drying out of the growing medium, you can use plastic wrap over the top of the trays. There are also commercially available clear plastic domes made to fit over the 10×20 standard trays. They are fairly reasonable in price so I just use those, but the plastic wrap will work just as well. Once the seeds are up, it is best to take off the domes in order to prevent an overly humid environment which can support the growth of some harmful fungi.  ‘Damping off’ is a common fungal disease that causes the death of many seedlings. It looks like the plant rots right where the little sprout goes into the soil. Once the plants have started to grow you can remove them from the heat mats. Leaving them on too long can be detrimental and cause them to get tall and leggy, and even flop over. Remember that the heat mats are only for speeding up germination time. At this point you can move your seed trays to an area with really bright light.  Avoid window sills as these are usually cold and drafty. If needed, supplement with overhead lighting set a few inches above the plants. Adjust the lights as the plants grow. Soon it will be time to harden off your transplants and set them out for a bountiful harvest.

The heat mat, probe and thermostat in action warming the soil to speed germination of some vegetable seeds. Note the clear dome to retain moisture. Photo by mrl2021.

by Matt Lisy, UConn 2021