You work very hard to grow the very best seedlings you can. So start them off right and give them best seedling mix you can. I have grown over 2,000 varieties of plants from seed and wrote the book Soil Science for Gardeners. In this post I review the best commercial and DIY seed starting mixes and give you my opinion on starting your babies off right.
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Seed Starting Mix vs Potting Mix
The terms potting soil, potting mix and potting compost are very confusing and their definition depends on where you live. in the UK the term “potting compost” is most common and consists of soil with added peat or decomposed wood. Australia uses the term “potting mix” and it’s usually wood based with no soil. In North America we are totally confused and use both potting mix and potting soil, interchangeably. Normally they are peat-based with no soil. I will use the following definitions in this article which is also the most common use in North America.
- Potting Mix – usually a soilless mix that is used for potting up plants or growing them in containers.
- Seed starting mix – a soilless mix for starting seeds.
For clarification of other soil terms like top soil, black soil, garden soil, compost, see Topsoil, Compost, Triple Mix – What’s the Difference?
There really is not much difference between a potting mix and a seed starting mix. The texture of a seed starting mix may be a bit finer and it might contain less fertilizer, but other than that they are similar. They generally have the same ingredients, but seedling mixes cost more.
Do You Need a Seed Starting Mix?
The short answer is no. A good quality potting mix works just as well as a seedling mix. I use Pro-Mix HP for everything indoors and have used it for 20+ years. I even use it for the extremely small streptocarpus seeds because its texture is fine enough for them.
Pro-Mix and other quality potting mixes have a fairly fine texture that works well. Some lower quality potting mixes have a lot of course material in them and they are not suited for starting seeds, except for large seed like peas and beans. If that is the only material you have it is a good idea to get a seed starting mix.
What is a Seed Starting Mix?
A seed starting mix is a soilless mix which means it contains no real soil, although it might contain a bit of sand for extra drainage. Its main ingredient is organic matter consisting of one of these:
- Peat moss – milled peat moss
- Coir – shredded coconut husk
- Decomposed wood – shredded wood products that have been composted
Most products in North America use peat moss but coir is becoming more popular. Decomposed wood is used more in potting mixes and low quality seed starting mixes. Its texture is not as fine as the other two products and therefore less suitable for small seeds. Fertilizing is also more complicated in it.
Other additives include:
- Perlite – holds water and improves drainage
- Vermiculite – holds water and improves drainage
- Wetting agent – helps the organic matter absorb water
- Fertilizer – feeds the seedlings (organic or synthetic)
- Compost – an organic form of fertilizer
- Mycorrhizal fungi – claimed to add biology to soil
- Lime – balances the acidic pH of peat moss
If you read the labels on seeding mix and understand what each of the above do, you quickly realize that the products are not that different from one another. The following sections will make this clear and help you select the best one.
Peat Moss vs Coir
Traditionally, all of these mixes used peat moss which grinds up to a very fine consistency that is perfect for seeds. Then people became concerned about using peat and damaging peat bogs. A seemingly good alternative is coir, which can also be ground fine and works. The reality is that a) using peat moss for potted plants is not really an environmental problem and b) coir has its own sustainability issues. From an environmental perspective coir is not better than peat moss. To understand this fully have a look at these articles.
Which is best for starting seeds? Peat moss works a bit better than coir and certainly in North America, it is the “more eco-friendly” option. It is also less expensive. If you live closer to India and Shri Lanka where coir is made, coir may be the better option.
Perlite vs Vermiculite
Both of these are inert material that hold water and improve drainage. Either one will do the job, but perlite is my preferred option.
For a full comparison of these, have a look at Perlite vs Vermiculite – Which Soil Additive is Better?
Do You Need both Perlite and Vermiculite?
Lots of DIY mixes contain both but since they do the same thing and have similar properties there is no reason to have both.
Organic vs Synthetic Fertilizer
Lots of people want to go organic, but when it comes to seed starting it’s not the best option for several reasons.
Organic fertilizer (compost, fish emulsion, kelp extract) needs to decompose in order to release the nutrients plants use. That works great in a garden, or even in potted house plants where the plant can wait until this decomposition takes place. But seedlings need access to nutrients right after gemination. They can’t wait for the organic matter to decompose and release nutrients, which can take years. Synthetic is available right away and therefore grows better seedlings.
Synthetic fertilizer also gives you better control. You know exactly which nutrients you are adding and the amount you are adding. With organic, you have no idea what the nutrient level is at any particular point in time. The vermicompost might be a 1.5-2-1, but how much of that is available to plants next week?
A big issue with organic is that many gardeners have been incorrectly duped into thinking that organic nutrients are somehow better and that synthetic is bad for plants. That is completely wrong. The resulting nutrients from decomposed organic material are identical to synthetic fertilizer. There is zero difference!
Organic fertilizer is better in a garden where you are looking for long term feeding and soil building, but not for seedlings in a pot.
Why Are Wetting Agents Added?
A wetting agent is a type of surfactant, similar to soap. It helps the inorganic water molecules absorb into the organic material (peat, coir). It makes watering easier and more thorough, especially after the mix dries out.
Wetting agents don’t affect plants directly, but they do make it easier to water them. It should be in the mix and can be either synthetic or organic (e.g. seaweed agar). Either type works.
Do The Mixes Have to Be Sterile?
Some manufacturers make a big deal about this. Is it important? What about a DIY solution – do you need to sterilize them?
This whole idea of sterility is a marketing gimmick. None of the soil-type bagged material is sterile – no matter what it says on the package. Some of the ingredients might have been heat treated to kill seeds or pathogens, but nothing in the manufacturing or packaging process is sterile. The minute you open the bag, microbes from the air start settling in the bag. Neither the pots, your hands or the seeds are sterile.
Gardening is not a sterile process and it does not need to be. I don’t even wash my reused pots to start seeds!
A bag of seed starting mix that claims to be sterile is just as likely to get damping off disease as one that does not make the claim. Fungus on seedlings is a cultural issue, not a soil sterility one.
Should the Mixes be Organic?
Since the major ingredient in all seed starting mixes is either peat moss or coir, they are all “organic”.
However, just because a package is labeled as “organic” does mean it is “certified organic“. An “organic” label does not really mean anything. A claim of being “certified organic” indicates the item can be used in certified organic operations. in this case the product will probably be OMRI (Organic Materials Review Institute) rated and show the OMRI symbol on the bag. Such a symbol means that there is no added synthetic fertilizer. It does not imply a quality product nor does it guarantee good seedling growth.
Is organic certification (OMRI) important? Only if you operate an organic certified farm. It really has zero value to a home gardener and your seeds don’t care. OMRI certification will NOT grow better seedlings, produce higher quality food or increase yield, nutrient levels or taste. It might however make you feel good because you are supporting the organic industry.
Chemical fertilizers do not harm plants or microbes. Find out more in 10 Fertilizer Myths That Will Save You Money
Do You Need Mycorrhizal Fungi
Mycorrhizal fungi are important organisms in the garden where they form beneficial associations with about 80% of the plants. They help provide nutrients for the plant and help it grow. That sounds great, but we are not dealing with soil in a garden. We start seeds in a very unnatural condition and we don’t want to rely on fungi to grow our plants. Fertilizer is a much better option.
Mycorrhizal fungi have become very popular and are the latest “fad” solution for all kinds of plant problems, but the science on this is fairly clear. Except in very specific disease situations they add very little value for your garden. They are even less useful in a seedless mix.
Does the Mix Affect Germination?
I see lots of claims that “brand A” gives better germination but that is mostly not true. Most seed that a beginning gardener tries to grow, including all vegetables and many annuals, germinate easily. They germinate on a piece of paper with just added water. That means the seed starting mix does not have an impact on either the germination rate or the percent germination.
However, each type of mix is a bit different and watering is slightly different for each one. If you don’t water enough or if you water too much it can affect germination. Matching the mix to your personal watering habits is a good idea but the mixes themselves do not affect germination.
Other factors can affect germination:
Are Potting Mixes Too Rich in Nutrients?
This is a myth I see frequently. First of all potting mixes don’t contain much fertilizer. Secondly, why would young seedlings need a lower level of nutrients than older seedlings? They don’t get that in nature where seedlings get the same nutrients as adult plants. They also don’t need lower nutrient levels in a pot.
I feed everything the same. Germinating seeds, young seedlings and older plants all get the same fertilizer.
Everything You Need to Know About Starting Seeds
Interested in a free seed starting course? Here is mine on YouTube:
Seed Starting Mix: Buying Guide
Here is a selection of products I would buy.
Best Overall Seed Starting Mix
Pro-Mix Potting Mix. Pro-mix also has a seed starting mix but you don’t need it. The potting mix can do everything you need for indoor planting.
Best Organic Certified Seed Starting Mix
Black Gold Seedling Mix. Popular and certified organic.
Best Non Peat Based Seed Starting Mix
Burpee Organic Coconut Coir + Perlite. An expandable coir brick plus some perlite all in one package.
DIY Seed Starting Mix
Lots of sites provide varying DIY solutions for making your own seed starting mix. Is this worthwhile? If you are new to growing from seeds, buy a mix. Once you have some experience and want to try different things go ahead and make your own.
Does a DIY Mix Save Money?
Not really. I know lot of sites say it does, but it doesn’t. The mistake these sites make is that they compare the cost of a small bag of pre-made mix with the cost of buying ingredients in bulk. Buying in bulk is much cheaper, but so is buying bulk seedling mix. I buy my Pro-Mix in 3.8 cu. ft. bags and they cost about the same as a similar sized bag of peat moss. You won’t save money making your own mix.
Why Use a DIY Mix?
It is important that you match the mix to your watering habits. You can match your watering habits to the mix and use a pre-made mix. Or if you don’t want to change the way you water than you can customize a mix to suit your conditions. Most people do better with more perlite to compensate for overwatering.
Best DIY Seed Starting Mix Recipe
If you are going to make your own mix, consider this recipe as a starting point and adjust for your situation.
- 3 parts Sphagnum peat moss or coir
- 1 part Perlite – use course material
- Lime (1 tbs/gallon for peat moss, not needed for coir)
You have two options for fertilizer. My preference is not to add it to the mix and just water with fertilizer solution. Alternatively, you can add some compost or worm castings to the mix.
Do not sterilize it. As explained above there is no such thing as sterile seedling/potting mix.