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Which is the best seed starting medium? An experiment

This year I’m growing almost all my vegetables from seeds. I’ve been alternating between several different seed starting mediums, and I wondered which was the best. So I did an experiment to find out.

The contenders

Rock wool plugs

Rock wool plugs are made by various companies. I used “A-OK starter plugs” by Grodan. These come in sheets of tear-away cubes that usually have a hole in the middle for inserting a seed or stem (for clones). If you look closely at the following photos, you’ll notice that two of my trays have cylindrical cups. I sliced the four corners off the cubes so they would fit more easily into the trays.

Store-bought starting medium

The store-bought starting medium was made by Black Gold.

Vermiculite & sphagnum moss

This is a classic seed starting recipe: mix of equal parts vermiculite and sphagnum moss. I’m going to call this “vermicu-moss”. The vermiculite I used was from an enormous bag I bought years ago. I don’t know where it originally came from, but I believe the granularity is classified as “medium”. I couldn’t find milled sphagnum moss so I used the long fibered kind typically used to line hanging baskets. This was a mistake. I was able to crumble it a bit while mixing it, but finer texture would have been better. After this experiment, I used a chef knife to mince the long fibered moss and this resulted in a much better texture.

Public service announcement: be careful about confusing true sphagnum moss with “sphagnum peat moss”. I recently learned that peat moss is kind of evil because they dredge it out of ancient peat bogs which has devastating environmental impacts. Don’t buy peat moss.

The experiment setup

I filled four trays with a variety of different vegetable seeds. For each seed, I did three rows, one for each of the three mediums. They were placed under two LED grow lights hanging approximately 12 inches above the trays. The lights were on an automatic timer alternating between 13 hours of light and 11 hours of dark. I don’t use the dome lids to avoid mold, so on the fifth day I carefully added a little water to each cell to offset evaporation.

From left to right, rock wool, store-bought starting medium, vermiculite + sphagnum moss

Results

The results were surprising to me. The results differed a bit between different seeds, but overall vermicu-moss outperformed the others. The results for each different seed are below.

Arugula

In three days, Arugula showed up first in the Black Gold and the vermicu-moss. There was also one seedling just breaking the surface of one of rock wool plugs (not pictured). So not much difference yet.

By the end of two weeks, all of the rock wool plugs produced a seedling, and all but one of the vermicu-moss. Only three of the Black Gold succeeded. I consider this one a tie between rock wool and vermicu-moss.

Cilantro

On the eight day, cilantro seeds seemed to prefer rock wool. The tiny seedlings are difficult to see in the photo, but they were visible in five of the six plugs. Only one seedling had emerged from the vermicu-moss and none from the Black Gold.

At the two week mark, rock wool was the winner with a healthy seedling in every plug and a straggler just breaking the surface of the last plug. Four of six seedlings succeeded in vermicu-moss and two of six in Black Gold. This one goes to rock wool with vermicu-moss a close second.

Cucumber

On the eight day, two of the organic cucumber seedlings emerged in the vermicu-moss.

At the two week mark, both vermicu-moss and the rock wool had two seedlings. The vermicu-moss seedlings were heartier and the Black Gold had none.

This one is a closer call, but in my opinion vermicu-moss narrowly defeated rock wool due to the increased speed and vitality.

Kale

On day five, almost every kale seed in the vermicu-moss sprouted, while only one had emerged from the rock wool and none from the Black Gold.

Kale seeds sprouted in vermicu-moss mix next to empty cells of rock wool and Black Gold.

At the two week mark, Russian kale seemed pretty happy in all the mediums. Rock wool and Black Gold both had only one miss. vermicu-moss had none.

Kohlrabi

On day four, a kohlrabi seedling appeared in each of the mediums. So far, no apparent difference.

At the two week mark, it was still very even. There was only one miss in the Black Gold. Every other seed succeeded. I consider this one a three-way tie.

Pole Beans

Kentucky Wonder Pole beans started to emerge on day eight. All three vermicu-moss seeds were visible, only one Black Gold, and zero rock wool were visible. These were seeds I collected from last year’s crop.

The trend continued up to the two week mark. All three vermicu-moss seeds were thriving. The Black Gold had one thriving seedling and another just breaking the surface. Rock wool was zero for three.

These results were so lopsided it was difficult to photograph! The following photo is the view from above “the canopy”.

Tomatoes

I tested several different varieties of tomato. At the two week mark, all three the mediums were reasonably successful.

Almost of the Brandywine seedlings succeeded except for one of the vermicu-moss. The one miss was probably a bad seed, but Black Gold seedlings were a bit taller and heartier.

All of the Golden Nugget seedlings in vermicu-moss succeeded. Rock wool was 3/5 and Black Gold was 2/5.

The Russian black plum tomato seeds looked slightly happier in Black Gold, with 4/5 succeeding. Vermicu-moss and rock wool were both 3/5 and slightly more frail.

All three of three San Marzano seeds succeeded in rock wool. Only two of three in vermicu-moss. And just one of three in Black Gold.

I also consider this one to be a tie between rock wool and vermicu-moss, with a fine showing by Black Gold as well.

Squash

I also tested a few different varieties of squash seeds.

After two weeks, the Waltham Butternut seeds clearly preferred the vermicu-moss, as all those seedlings thrived. Black Gold and rock wool had only three and two seedlings respectively.

Winter squash seeds also preferred vermicu-moss. After two weeks, it produced three vigorous seedlings and a fourth latecomer. Black Gold had just one. Rock wool had three seedlings, which were smaller and less hearty.

Across all the varieties of squash, vermicu-moss is the clear winner.

Conclusion

Looking across all the seeds I tested, the vermiculite and sphagnum moss mixture either outperformed the commercial products, or was a very close second. On top of these findings, there are a few more things I really like about “vermicu-moss”.

  1. It’s the cheapest! Vermiculite is very inexpensive, especially if you purchase in bulk. True sphagnum moss (that isn’t actually evil peat moss) can be difficult to find, but it is not expensive. It also grows naturally in many parts of the world, so you could theoretically gather it for free and then dry it yourself.
  2. The components are easy to stockpile. You can buy a big sack of vermiculite that will last a lifetime. And sphagnum moss should keep for years if stored somewhere dry.
  3. Easy to transplant. Rock wool plugs don’t integrate into the surrounding soil when transplanted to a raised bed or larger container. They tend to contain the roots and so it takes much longer for the seedling to get established. They also seem to dry out faster than the surrounding soil, so I sometimes have to water them frequently or else the seedlings shrivel up. I don’t have this problem with vermicu-moss or Black Gold.

I’m going to keep experimenting, but vermicu-moss is my new favorite starting medium. I hope you found this helpful. If you know of an even better seed starting medium, let me know!