5 Heat Mat Mistakes to Avoid When Starting Seeds This Season
Ensure that your heated germination mat enhances seed starting rather than hindering it. In this article, former organic farmer Logan Hailey explains the most common mistakes and how to avoid them so you can grow strong, healthy seedlings.
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A seed-starting heat mat is one of the best ways to ensure even and successful germination. If your home, unheated greenhouse, or nursery space is a little chilly—and you don’t want to heat a large area—these warm electric mats are a great way to create a microclimate for emerging seedlings.
Warm-weather crops are particularly vulnerable to cold soil temperatures while sprouting and adding bottom heat to a tray can help speed up the process and increase the number of seedlings that germinate.
However, germination mats can cause problems when used incorrectly. If your trays overheat or seedlings are left in place for too long, you could accidentally harm your early-season garden transplants. Here are the five most common mistakes to avoid when starting seeds on a heat mat and what you should do instead.
Epic Seed Starting Heat Mat
- Durable, Spill-Resistant PVC Construction
- Ideal Size for Universal and 1020 Seed Trays
- Energy-Efficient 20W Power for Optimal Heating
- Promotes Faster Germination and Plant Growth
5 Mistakes to Avoid When Starting Seeds on a Heat Mat
Increasing soil temperature is the best way to ensure great germination when starting warm-weather crops indoors. Species like tomatoes, eggplant, zucchini, peppers, and melons typically require soil temperatures of at least 60°F (16°C) to sprout. If the trays or ambient temperatures are too cool, the seeds may germinate erratically or fail to sprout altogether. This can result in a huge waste of time and money.
Thankfully, these mats make seed starting much easier and more accessible. However, they are not without issues. Be sure to avoid these mistakes so your warmth-loving seeds grow into thriving, robust transplants.
Forgetting to Remove Seedlings
Bottom heat is extremely advantageous for sprouting seeds, but it is a mistake to leave seedlings on the mat forever. Many warmth-loving crops like melons and tomatoes love hot soil while germinating, but once they sprout, they should be moved to a regular nursery table where they can continue growing.
Leaving seedlings in place can scorch the roots and lead to weak, lanky growth. Prolonged exposure to bottom heat can also cause fungal disease issues when combined with regular irrigation. Additionally, you need to harden off seedlings so they can develop stronger stems and adjust to real-world conditions outside in the garden.
How to Avoid It
Keeping a heat mat on for too long is sort of like babying a teenager—eventually, young plants need to be removed from their cozy nursery and grow up into robust crops without depending on bottom warmth.
Once 50-80% of seeds sprout, remove the tray from the mat. Check your mats daily for signs of germination. Most garden crops will grow two cotyledons (the initial sprouted leaves), indicating that they are ready to be removed from the warmth.
Wasting Space
There is only so much space on a mat, so you want to be sure to maximize the amount of seedlings you can sow on top of them.
How to Avoid It
Avoid wasting space with oversized cell trays when you can start in smaller cells and up-pot later. You can also reserve germination mats solely for crops that demand warm soil temperatures, such as peppers, tomatoes, squash, zucchini, cucumbers, eggplant, and melons.
Overheating
Most germination mats can increase soil temperatures 10-20°F above the air temperature. They do this with electrical coils inside PVC or rubber mats that emit warmth upward to the bottom of seedling trays, where it can directly warm the root zone of your crops. Adjustable thermostats allow you to customize how much heat is delivered to your seedlings.
How to Avoid It
If you purchase a quality heat mat like our Epic Seed Starting Heat Mat, you don’t have to worry as much about setting the temperature. These mats have built-in thermometers to warm the soil just enough without overheating it. The low-energy 20W power source provides steady plant growth support at a consistent level.
On the other hand, if you are using high-powered mats with adjustable displays, check them regularly to ensure they align with your desired conditions for specific seeds.
Here are some average seed germination temperatures for heat-loving crops that are commonly started on heated germination mats:
- Melons: 60-85°F (16-29°C)
- Okra: 85-95°F (29-35°C)
- Tomatoes: 65-85°F (18-29°C)
- Cucumbers: 65-95°F (18-35°C)
- Eggplant: 75-85°F (24-29°C)
- Peppers: 65-75°F (18-24°C)
- Pumpkins and Squash: 85-95°F (29-35°C)
Remember, a heat mat raises soil temperatures, not ambient temperatures. Use a soil thermometer probe to check the warmth of the dirt inside your trays.
Placing on a Cold Surface
Placing a germination mat on a cold surface near window drafts or air conditioning units can dramatically set back your plants’ growth. The chill of the table will counteract the warm temperatures of the mat and potentially make it ineffective, leading to reduced germination or zero sprouts from species that demand high temperatures.
How to Avoid It
Build your seed starting setup in a warm area of your home using wooden or slotted shelving. I’ve found that metal hardware shelving with a wooden plank across it works great, but there are also shelves customized just for starting seedlings. For a smaller setup, try a solid surface like your kitchen counter, a wide windowsill, or any plastic portable table.
Forgetting to Acclimate
Many seedlings require warmth to germinate, but they don’t need excess warmth once they’ve sprouted. Nonetheless, they cannot tolerate huge temperature swings either. It’s a mistake to remove plants from heat mats and then suddenly expose young plants to drastic temperature changes, as this can shock them and set back their growth.
How to Avoid It
When removing trays from germination mats, gradually acclimate them to lower temperatures by moving them to another indoor table without heating for a few days. Then, you can proceed with up-potting and hardening off outdoors as needed, depending on the species.
Low-Quality Mat
A cheap, low-quality mat will fail to provide the consistent bottom warmth that can actually make a difference for your plants. It is also very frustrating and wasteful to watch cheap electronics break within a few weeks of purchase, only to yield more plastic in landfills and more expenses added to your garden budget.
How to Avoid It
It is worthwhile to invest in a quality mat from the start so you don’t have to worry about issues later. Look for mats that can increase the soil temperatures by at least 10-20 degrees by plugging into a standard outlet.
Water-proof surface covering is a must, and adequate sizing for a 1020 bottom tray is ideal for maximizing space. Energy efficiency is a bonus if you plan to use bottom heating for most of the season.
