If you are trying to germinate your seeds or increase the speed that they germinate, don’t be tempted to reach for a bottle of fertilizer. Fertilizer has no real impact on seed germination.
If you’ve tried germinating seeds but they never sprouted, there are many possible reasons why. They could be too old or have succumbed to mold. Some have specific requirements, like a period of cold treatment (stratification) or they require light to germinate. And others still, like cilantro/coriander seeds and carrot seeds are just naturally more tricky to germinate, even for experienced gardeners. And if you planted them outside, it’s also possible that some critter ate them.
Keep reading to know more about the factors of seed germination and the effect (if any) of fertilizer on seed germination. Once you’re aware of the specific conditions required to germinate your favorite flowers and vegetables, it’s really easy to germinate them.
Do Seeds Need Fertilizer to Germinate?
Seeds do not need fertilizer to germinate. Since seeds contain all the nutrients a seedling needs to emerge and grow its first few leaves, they can germinate without any additional nutrients.
Moisture, light/darkness, warm/cold temperatures, and oxygen are what’s necessary for germination. Seeds already come packed with enough stored energy to push out the first root down and first shoot up to the surface. Essentially, every seed is self-sufficient up until the first true leaves appear.
Does Fertilizer Affect Seed Germination at All?
Fertilizer has no effect on seed germination if used in small amounts, but can negatively affect seed germination if you use too much fertilizer.
Since seeds require no external sources of nutrition to germinate, it’s not recommended to apply fertilizer to seeds. In fact, it can reduce germination rates.
The chemical components of fertilizers are comprised mainly of different salts (e.g. calcium chloride, magnesium sulfate, ammonium nitrate, etc.). In high amounts, these fertilizer salts can harm seeds and the first tender roots the same way they “burn” mature plant roots if overused.
Instead, save your fertilizer for after seedlings emerge and the first true leaves appear, then apply a dilute fertilizer solution.
What Things Do Seeds Need to Germinate?
Generally, seeds have a combination of temperature, water, light, and oxygen requirements for germination, but for most species of plants, gardeners should be concerned with water, temperature, and sometimes light conditions.
Seeds need water in order to sprout. Water is what softens the seed coat and hydrates the seeds, activating enzymes which start breaking down the seed’s energy stores, converting starches into sugar. The hydrated embryonic cells inside the seed begin to divide and germination begins. Large seeds absorb more water, which is why many people soak bean and pea seeds overnight to before planting to speed up germination, although it isn’t necessary if you have well-moistened soil.
Temperature is also very important factor. For most vegetables, fruits, and flowers, anywhere between 68°F to 86°F (20-30°C) is optimal for germinating seeds. Some plants are more picky about germination temperature. For instance, beans really prefer to sprout at around 80°F (27°C), and will germinate significantly more poorly below 68°F., whereas cool weather vegetables like arugula, lettuce, spinach, and kale, can germinate even at near-freezing temperatures. However, colder temperatures slow down germination and seedling growth, even for cold-loving crops.
If starting seeds indoors, you can use a temperature-controlled heating mat to increase germination rate.
Do All Seeds Need Warmth to Germinate?
While most seeds do need warmth, they can germinate in cooler temperatures, albeit more slowly. However, some seeds require a period of exposure to cold temperatures before they can sprout, called stratification.
Some seeds need a period of cold temperatures in order to break dormancy and germinate. Stratification is an evolved survival mechanism that keeps seeds in nature from sprouting in the fall before a killing frost.
Most vegetables don’t require cold stratification, although some like lettuce might have improved germination rates. However, for some fruit trees (apples, pears, peaches, cherries, etc.) and perennial flowers (St. John’s wort, roses, hibiscus, primrose, violets, etc.) have much better germination rates after a period of cold treatment.
Some stratified seeds also need to germinate in the cold, but many can be planted after the cold period.
Different seeds have different light requirements to germinate. Most seeds germinate best buried in the soil but will still sprout with some light, however a few (such as plants in the onion family) require darkness to germinate.
Surprisingly, there are a few species of plants that need sunlight to germinate. Examples of this are lettuce, celery, poppies, and impatiens. These are usually plants with very tiny seeds, which don’t have enough stored energy to grow when planted deep in the soil. For these seeds, either surface sow or very lightly cover with a thin layer of soil and germinate them outside or indoors next to a window or under a grow light.
All seedlings, once sprouted, require light.
Plants produce oxygen via photosynthesis but they also breathe oxygen as well. Seeds need oxygen to germinate properly. In most cases, this is not something most gardeners need to worry about, but it’s important to germinate seeds in a moist environment but not completely waterlogged, otherwise the seeds may not germinate or if they do, the sprouts could drown before they can get established.
When Should I Start Fertilizing My Seedlings?
As a rule of thumb, you can start fertilizing seedlings with a very dilute liquid fertilizer after the first set of true leaves appear.
When seedlings emerge, the first set of leaves, called seed leaves or cotyledons, open up so photosynthesis can begin. These cotyledons are small and narrow, and don’t look like regular leaves. However, the next set of leaves that appear will look more like the normal leaves of the plant – these are called true leaves.
Note: If you are using potting mix or seed starting mix with fertilizer included (it should say so on the bag) then fertilizer is not necessary at the seedling stage, but you can add diluted liquid fertilizer after a few weeks if you will be letting your seedlings grow more before transplanting.