Most seeds will germinate over quite a wide range of soil temperatures but the speed of germination will vary. Too cold and they'll be very slow to sprout and too hot will also reduce the speed of germination. Far too cold or hot and they'll just fail.
Soaking your seeds in hot water held to a specific temperature will also break down the seed coating. But this one is a double-edged sword. Soaking seeds in hot water will help speed up germination but can come at the cost of lower germination rates.
Their germination doesn't start to drop off until the soil gets to be over 90°F. The optimum temperature for lettuce is around 65°F and germination drops off rapidly after the soil heats up to the mid seventies. Spinach does best at 70°F, but by the time the soil is in the mid-80s, forget about it.
Temperature is a critical environmental factor in seed germination [25,26]. The pace and rate of germination, which govern water absorption, may be affected by temperatures above or below the optimal range. Under optimal conditions, the absorption process is fast.
Vegetable seeds will germinate at a wide range of soil temperatures. Most seeds prefer to germinate in a range between 60 and 90 degrees. Some seeds are so sensitive to soil temps that they won't germinate if the soil is too hot or cold.
For many garden plants in fact, a soil temperature below 50°F can be a problem. With temperatures in the 40s, seeds will readily absorb water, but not start to grow. This creates an opportunity for disease and rot, which can result in damping-off or poor growth.
The best temperature to germinate most seeds is about 80°F (27°C). Many commonly-planted garden seeds will germinate in a week or two when the temperature is in an optimal range between approximately 65-85°F (18-29°C). Germination time and temperature varies between seed species and even cultivars.
Optimal conditions are 85°F day, 70°F night with diffuse light and high moisture. It is the policy of the University of California (UC) and the UC Division of Agriculture & Natural Resources not to engage in discrimination against or harassment of any person in any of its programs or activities.
Most of the seeds germinate best in dark environments. The presence of light tends to inhibit their growth. The light decomposes carbonic acid gas and expels oxygen which leads the seed to harden. These gases are key factors that promote germination.
All seeds need water, oxygen, and proper temperature in order to germinate. Some seeds require proper light also. Some germinate better in full light while others require darkness to germinate. When a seed is exposed to the proper conditions, water and oxygen are taken in through the seed coat.
Light is not necessary for a seed to germinate, no. The majority of seeds grow most effectively in the dark. Light, which is essential for seedling growth, may actually hinder the germination process. The three primary and necessary conditions for a seed to germinate are water, oxygen, and temperature.
Both under watering and over watering could cause seeds not to get enough oxygen for germination. Under watering may have caused problems with the coating not breaking down. Over watering may have caused the soil to become waterlogged and compacted. Compactions makes it more difficult for oxygen to get through.
Until seeds have sprouted, keep the seed bed moist, never allowing it to dry out. Water with a fine-spray hose nozzle or watering can which will provide a fine misty spray and not wash away the soil. Water often enough (usually about once a day) so that the soil surface never dries out, but remains constantly moist.
Too much or too little water is the most likely reason for seeds not germinating. With too little or no water, seeds remain dormant.
Some seeds like Allium 'Purple Sensation' and Eryngium giganteum need a long period of cold-moist stratification with alternating temperatures. These are the most difficult seeds to germinate.
They actually prefer long nights during germination. So you can start them indoors under plastic, and remove the plastic and place them under light after they've sprouted.
The beginning of the growth of a seed into a seedling is known as germination. All seeds need water, oxygen and the right temperature to germinate.
Many seeds will not germinate at all if the soil gets too hot. During periods of extreme heat, one option is to start these seeds indoors under lights, and then transplant them into the garden after hardening them off (gradually adjusting the plants to direct sunlight and wind).
So, how hot IS too hot for plants? The general answer is around 90 degrees F, with some exceptions to the rule. This means that when temps rise above 90 and remain there for a lengthy spell: Leaves wilt.
High temperature and high relative humidity will reduce seed germination and vigor. Do not store seed in areas that have high temperatures (greater than 70°F) or where humidity values are greater than 60%. The ideal storage temperature for seeds is 35-40°F with a relative humidity of less than 40%.
Excessive heat can cause seeds to dry out too quickly, preventing germination. In order for seeds to germinate, oxygen availability may be reduced by high temperatures. High temperatures can also harm seedlings, resulting in stunted growth or even death.