For the past few years, more and more companies are offering coir and coir-based products for seed starting. Coir is a natural fiber that comes from the husk of a coconut.
I understand coir’s appeal. It’s natural, long-lasting and very sustainable. It can be compressed into blocks that ship easily. The pH of coir is neutral so when you add it to soil, it doesn’t affect the pH like peat moss, which is very acidic. The other selling point for coir is that it holds water very well.
This last point is why I don’t use coir-based mediums for growing seedlings. A few years ago I tried it as the soilless mix for growing tomatoes, peppers and eggplants. After a few weeks I noticed something. The tomatoes were fine. The eggplants were growing slowly but they seemed OK. The problem was the peppers. The small plants started to lose their lower leaves and the leaves that remained were clorotic (yellow). Like many gardeners, I tried fertilizer as the solution (doesn’t fertilizer fix everything?) but it didn’t help. The peppers were alive but they were sad little plants.
I did some checking online and found the problem. If peppers are kept too wet, they’ll drop their lower leaves and the remaining leaves will become clorotic. I’d never had this problem before so what was different?
Coir holds much more water that peat moss and I’d been watering the plants as if they were growing in peat. The tomatoes were fine because they’re not as sensitive to overwatering. The eggplants were a little stunted but the peppers were down-right pitiful because there was too much moisture around their roots. I cut back a lot on watering and everything improved.
I’ve been growing seedlings since I was in grade school. With a peat-based product, I know when the plants need water just by looking at the surface of the mix. When you use coir, you have to water far less and even when the surface looks dry, it often isn’t. For people who sometimes forget to water their plants, this would be the reason to use a seed starting mix with coir. For me, it just led to overwatering and defoliated, clorotic peppers.
I’m not saying coir is bad. It’s a good soil amendment for outdoor beds. It’s great for outdoor pots which can dry out quickly. But for inside seed starting, it holds too much water for me. This old dog isn’t ready to learn the new trick of growing seedlings in coir! I’ll stick with peat moss.