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Things you should never add to seed starting mixtures 

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Have you ever wondered what you should add to your seed starting mixes? Whether using a coconut coir or peat moss seed starting mix there are some things you should never add. This gardening in Canada article looks at 3 thing to never add when seed starting.

If you are new to this blog my name is Ashley and I am a soil scientist. I am located in a Canadian Zone 3 and a USDA Zone 4. I write articles, make YouTube videos, Instagram & Facebook posts all designed for Canadians and Cold Climate gardeners using science-based methods. If you are looking for anything specific be sure to let me know in the comments down below.

Beneficial Microbes & Fungi : Never Add When Seed Starting

Beneficial microbes and fungi are a popular topic in the gardening community right now. This is rightfully so, they are great for helping reduce water usage, grabbing and cellulose in more nutrients, and helping plants fight disease. However, using them at the time of seed starting is not advised.

If you’re an experienced gardener and want to try out using bacterial additives at the time of seed starting please do. However if you are a beginner gardener I would avoid any form of Myco fungi at seed starting. Beneficial fungi at the time of seeding may cause excess fuzz. 

This excess fuzz and beginner Gardner can be concerning. And of course fun dry is one of the number one causes for damping off. If you want to avoid this and reduce your stress do not use a fungi additive at the time of seating. Instead save your fungi additions for the bumping up process or at transplant.

However when it comes to beneficial bacteria being added go wild. Beginner or an expert will see the benefits of adding bacteria to a seed starting mix. This is particularly true if you’re using a rhizobium bacteria with legumes. This will cause a symbiotic effect resulting in higher levels of atmospheric nitrogen capture. This directly affects your yields in a positive fashion.

Perlite & Pumice : Never Add When Seed Starting

This may seem odd but adding excessive amounts of perlite or pumice can harm your seedlings. Naturally seed mixes will come with a base level of perlite and this is fine. Adding any extra will not serve you any benefits.

There’s a big reason for this and it comes down to root exposure to air. If you are not yet familiar with the concept of air pruning. I encourage you to look this up. Essentially what happens is the root is exposed to air and ultimately dies off. This causes a branching effect which in an adult plant is beneficial. The branching means more root hairs and ultimately more water and nutrient capture. 

However when it comes to ceilings we do not want this to take place. Initially we have very few routes at our disposal. This means air pruning in any capacity is not a good idea. The plant has very little means of producing new energy. This is due to the lack of upper biomass.

To help support healthy root development without any excess stress. We want to avoid the use of aerating products such as pumice, perlite, volcanic rock, leca. Save these products for the bumping up process or at transplant. 

Compost : Choose Carefully

This is another one that may seem odd. And it may even become more odd sounding as more and more people start using soil blockers. But there is a good reason for not using compost. 

Exception to rule is vericompost or worm castings as some people like to call them. This technically is more of a manure that it is an actual compost. This means these rules do not apply. But this rule does apply to any compost made from plant material. This includes leaf molds, vegetable compost, wood chip compost, mushroom compost, etc. 

With any compost we still have a decomposition process taking place. This means there’s a likelihood you’re introducing both bacteria and fungi. Despite our best belief that are compost only has beneficial bacteria this may not always be the case. The use to compost commonly will result in excess fungal growth and the potential for damping off.

The second big reason why you should not use compost in a seats starting mix comes down to alleopathy. Alleopathy is an incredibly fancy word for the concept of plants emitting chemicals to suppress seed germination. Essentially what happens in the plant kingdom is plants have the ability to release certain chemicals as they are decomposing. The purpose of this is to ensure the Genetic line of their seeds go on unencumbered by competition.

The chemicals released results in suppression of other scenes germinating. This means if we use a compost that is not yet fully digested we run the risk of suppressing germination. If you’re dead set on using compost then I suggest you do a quick test.

Testing to see if compost is ready:

  1. Pick up some inexpensive seeds from the store.
  2. Count the total number of seeds you are about to plant. Pro tip: make it an even number it’ll make your life a lot easier.
  3. Plant those seeds in straight compost you intend to use for a seed starting mix.
  4. Wait to see how many of those seeds germinate.

A good germination test generally yields about 80% germination. This means if you plant 10 seeds eight seeds should turn into plants. If it is any lower than 80% gerbernation rate there could be a risk that your compost is not yet ready for seed starting. 

This rule and test can be used indoors and outside. So if you intend to add compost to the garden this year maybe try this test out. It will help ensure proper germination directly in the seed bed. The good news is that if the compost does not pass the test you can still use it with your adult plants. This means perennials or well established annuals can be top dressed with this compost. 

This arguably may help with your weed suppression. Because it has the ability to suppress germination of seeds and very well may suppress any weed seeds from germinating.

There you have it the three things you should never add to a seed starting mix. I hope this little bit of plant science was able to help you out. For more info be sure to check me out over on YouTube or actually show you some of the techniques I use.

3 Things To Never Add When Seed Starting
3 Things To Never Add When Seed Starting
Gardening In Canada