Seed Germination Temperature Chart

General Information

The minimum temperature displayed does not means that seeds will take to germination at this temperature but only that they have a possibility of doing so. While seeds can germinate across a range of temperatures the optimal range is key. When the temperature falls beneath optimum range the strike rate drops considerably, and the length of time increases for germination. The longer germination takes the more susceptible seeds are to disease. For any seed, the optimal soil temperature will provide not only faster germination than any other temperature, but the maximum number of seeds will do so.

N.B – some seeds, for example, Celery, have different day and night temperatures for germination.

Considerations for Vegetable Seeds

Daylight and Plant Growth

Vegetables coordinate their growth through daylight. This means they allocate their internal resources and timing according to the light available to them. Given too little light and they may grow thin and tall, too much light and they may deteriorate fearing the onset of autumn. Different vegetables have different demands of light. Onions demand a minimum 14 hours of daylight for their bulbs to bulk. Planting onions around mid-April will provide them this, but earlier or too late will result in no growth or decay. Similarly, fruiting vegetables need 16 hours of daylight to produce. A warm March environment will not suffice, and their likelihood of fruiting will decrease.

Soil Temperature and Germination

The amount of daylight and heat from the sun are the two factors influencing soil temperature. For seeds to grow the soil must be warm enough for them. Outside growing leaves the process entirely dependent upon the weather. For outdoor germination, consider using a propagator, cold frame or growing in a greenhouse. Grow tunnels, cloches and plastic sheeting are also effective. A solid indicator for sowing seeds is the feel of the soil. If it is cold to touch, sowing and planting out crops isn’t viable yet. On the other hand, weed growth is a good indicator for the soil temperature picking up. An allotment alive with dandelions and nettles is a sign planting out and sowing is an option.

Measuring Temperature – Indoors & Outdoors

When starting seed germination indoors it’s worth investing in equipment to safeguard ideal temperatures. A seed-starting heat mat with a thermostat is an effective way to keep the potting mix in the best temperature range. For either indoors or outdoors, using a soil thermometer is only advisable. This being said, a standard digital kitchen thermometer will also work. Once seeds have sprouted the temperature can be lowered by around 5 degrees. Seeds require warmer temperatures to sprout than grow.

General Tips For Gemination

Keep Records

Record keeping is an important yet easily overlooked aspect of seed germination. A propagation journal is wholly advisable whether you are planting just a few plants in your garden at home or caring for a large nursery. A journal should include which seeds are sown, the germination date, their success rate and when seedlings are ready for transplanting. Having a record at what went well and what went wrong can guide future sowing and make sure grows happen under optimal conditions.  

Use Quality Seeds

Seeds must be viable in order to germinate. They need to be fresh and of the highest quality. Low-quality and old seeds can give inconsistent results, particularly if their storage hasn’t been great. Purchasing from recommended suppliers and retailers is only advisable. Check sow-by and expiry dates on the packaging. This is as important for saving seeds as it is for sowing.  

Store Seeds Properly

The fragility of seeds means that if they are not stored adequately, they will fall into decay. Different seeds hold different characteristics, some can last centuries stored correctly, while others will decline irrespectively. To keep seeds dormant, they need to be stored in a dark, cool location with low humidity. A refrigerator works well for this. Labelling them with their name, source and year, keeping them in a small reclosable bag to be stored in tupperware is best.  

When ready to sow, the viability of seeds is easily tested. Soak them in water for a few hours. Dead seeds float to the surface, living ones will sink to the bottom. This is a test for larger seeds in particular, but not exclusively.  

Pre-Treatment

Some seeds will have their strike rate greatly improved with some special treatment before sowing. Certain pre-treatments including soaking, pre-chilling and scarification can activate enzymes and break down seed coatings. These only aid germination. Not all seeds require pre-treatment, check which ones do or don't before proceeding.  

Avoid Sowing Densely

Planting extra seeds to guarantee you have plants for next season is standard. However, dense sowing, with a cluster of seeds can result in weak, stunted and spindly growth. It's important to thin your seeds. When using seed trays only plant 2 or 3 seeds per cell. Most vegetables can be thinned to a seedling per cell. If using fine, small seeds, a hand seeder can stop clustering too many seeds in a single area.  

Use Flat, Wide Containers

Plastic containers and pots are best for seed starting, they have a better retention of moisture. It is preferable to use wide, shallow containers that prevent both excessive moisture and overcrowding around young, fragile roots. This being said, plenty of recycled plastic containers are fit for task from margarine tubs to empty yogurt pots. Just ensure holes are poked in the bottom for drainage. Make sure any container used is sanitized. To do this, soak in a 10% bleach solution for 15 minutes and air dry it.  

Sow at the Correct Depth

Seeds must be appropriately sown at their ideal depth for proper germination. You want to avoid burying the seeds in a thick layer of soil. Nor do you want to bury them too deep, this will stop seeds from having enough energy to reach light. Alternatively, seeds that aren’t covered sufficiently will be vulnerable to the elements. Seeds planted too shallow can be dislodged, blown away or dried out.  

Generally, seeds need sowing twice as deep as their size.  

• Large seeds can be sown up to an inch deep in soil. 

• Medium seeds can be sown in hole twice their size.  

• Fine, tiny seeds only require a thin layer of soil over the top.  

Well-Draining, Aerated Soil

Drainage is paramount for successful germination, it stops seeds from rotting in the soil. Water should be able to move freely through the seedling tray. A well-draining soil mix is ideal as it not only deals with moisture it provides better aeration. More oxygen between particles helps seeds breathe. Uneven germination is likely down to a heavy and poorly draining soil mix.   

Seek a soil mix formulated for seed starting. These will differ from garden bed or potting mixes and will take drainage demands into account. Ingredients worth seeking are:  

• Sieved compost 

• Perlite 

• Vermiculite 

• Coco coir 

• Peat moss 

• Vermicompost 

An ideal soil for seed germination will be nutrient-rich, loose and well-draining. You should also ensure the soil pH is appropriate for the type of plants you are growing.