Table of Contents
Environmental Conditions for Successful Seed Germination
Let us cover the four basic environmental conditions required for plants to thrive as well as details from each condition for seeds to germinate successfully.
Ideal Temperature and Humidity for Seed Germination
Light, temperature, moisture / humidity, and oxygen are the non-living foundations of germination and plant growth in general. Besides these, fungus, bacteria, and other single cell organisms make up the biotic foundation of growth. Biotic being living factors, abiotic being non-living factors.
There is a hierarchy of abiotic environmental conditions, and it starts with temperature. Seeds can still germinate without light, little to no moisture, and in low or high humidity, but must be in the right temperature range to germinate. It is the warming soil temperature that initiates a series of biological responses that triggers the hormones associated with seed germination.
Plants are sessile organisms; they cannot migrate to find the right conditions. Instead, they have rapid response mechanisms to initiate germination when the conditions are exactly right. Due to this, some seeds are equipped to wait longer for the conditions to be exactly right (viability) while others cannot wait extended periods of time (name seed with low viability length). Onions, okra, and peppers are examples of seeds that drop viability of germination over 1-2 years. They will still germinate, but the percentage of germination has a likelihood of decreasing.
Temperature for Seedlings After Germination
After germination when seedlings are growing indoors, 35-60% humidity will suffice and 45-55% is ideal. For germination however, humidity levels need to be higher, over 75%, to ensure proper germination. If your germination room does not have 75% humidity, humidity domes are necessary for getting the humidity levels high enough and to localize it directly around your seedling tray. If you see condensation build up, then that is a good thing.
Humidity Levels for Seedlings: How Much Is Needed?
If your room is not in the correct humidity range, a humidifier or de-humidifier can help keep you within the right range throughout the season indoors.
50-75 degrees Fahrenheit is the right range for cool season crops, so as long your seedling room is within this range, heat mats are not necessary. Warm season crops on the other hand will require heat mats to maintain a temperature of 80-85 degrees Fahrenheit. One tool that is important to have in your seedling room is an indoor digital thermometer that tells us both temperature and humidity.
Supplemental Lighting for Indoor Seed Starting
Supplemental lighting is the next important environmental factor to consider. T5 fluorescent bulb fixtures are excellent to use for germination.
One light fixture has four bulbs and will suffice for four trays. T5 fluorescent provides a full spectrum of light, specializing in the white/bluish part of the spectrum, great for germination.
A good rule to follow is to position the fixtures six inches above each tray and raise them up one inch as your seedlings grow one inch at a time. This of course is not necessary if you are starting plants that stay small for an extended period. This can be the same for warm season crops.
Water and Oxygen Requirements for Germination
Lastly, let us get into how to keep the trays properly watered and with good access to oxygen. A good rule to follow is to water once a day but to observe how fast the trays dry out on a day-to-day basis. When using a humidity dome, If the humidity is higher and the trays are still well saturated on any given day, spot watering, or skipping a day completely will suffice.
For smaller diameter cell trays (128, 188, 200) a watering can is too powerful and will splash soil and seed out of your tray. Instead, a backpack sprayer that has a fine spray nozzle for overhead watering and easy access to the tray can be a great option. Watering or spraying every day will suffice, if evaporation is high like it is in dryer climates, then watering twice a day is necessary.
Another effective type of watering is bottom watering. After two weeks and the seedlings have emerged, bottom watering is effective. With a watering can, only fill until the bottom of the cell tray barely touches the water.
Check in on the level of the water in the bottom tray and fill when you notice the level has gone down well below the cell tray.
Airflow for Seedlings: Preventing Disease and Strengthening Stems
Fresh air is important for people, and it is important for plants as well. Lack of oxygen in and around your seedlings breeds airborne and soilborne diseases which proliferate in anaerobic conditions. Anaerobic environments are environments that lack oxygen all together, and pair that with too much water, well then you will start seeing your seedlings mysteriously start to wilt and fall. This occurrence is known as dampening off and can be the result of anaerobic pathogens. You need to focus on aerobic environments by ensuring that your plants have access to fresh air.
Once you see at least 25% of your tray is germinated place a fan in the direction of your trays for proper air movement to strengthen your seedlings and fight off air borne plant diseases. Constant air flow is not necessary, so keeping the fans on shorter intervals via timers is a helpful solution.
Wind, like in nature, strengthens your seedlings through a plant’s (healthy) reactions to environmental stresses called thigmomorphogenesis. When managed properly, adding airflow on a timer will initiate thigmomorphogenesis correctly and strengthen the cell walls within in your plant to prepare it for any other stresses. Putting your fans on a timer will also save energy.
From everyone at Harris Seeds, happy planting.
Drew Hundelt
Product Manager & Content Specialist
Since stepping foot on his first urban farm in 2012, Drew found a passion that would eventually lead him here at Harris Seeds, Garden Trends Inc. Growing in multiple environments ranging from western New York to the bay area of California, and many environments in between, Drew has been and continues to build food production systems that yields food and community. Drew’s favorite projects so far in his career include carrying out a 12-acre trial of regenerative winter wheat production in western Pennsylvania and creating a horticultural program with three acres of market gardens, food forests, and greenhouses in the urban landscapes of Denver.
