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Pros and Cons of Polymer-Bound Seedling Plugs

Wanted: the perfect media

People have been trying to start seeds in weird stuff for a long time. From gravel to packing peanuts, just about everything has been tried.

The great thing about seeds is that they come pre-packaged in a neat shell with all the nutrients they need to germinate and thrive for the first few weeks of their lives.

This means that the requirements of a seeding material are few. Most seeds require only three things from their media:

  • consistent moisture (but not wetness)
  • aeration (oxygen throughout the media)
  • lack of disease (not recycled from diseased crops)

(Of course, seedlings also require light and warmth, but these aren’t connected to the type of media.)

With these three things, almost anything will do for seedlings—but this doesn’t mean that anything will do for you. Different types of systems and the personal preference of farm managers will determine other characteristics of the seedling media you choose.

For example, you’ll need a media heavy enough to provide stability, but light enough to handle and move. You may care deeply about using sustainable materials, so you want something you can either recycle or reuse. These farmer-specific goals will whittle down your media options.

Recently, flexible or polymer-bound plugs have risen in popularity, especially for systems with recirculating irrigation (like hydroponic systems) and limited space. There are several nice qualities that bring them to the top of the list.

Pro: clean and neat

Polymer-bound plugs incorporate an organic material like peat or coco coir and bind it together with a polymer—picture a rubbery glue. This makes the plug very contained.

Unlike a soil, plain peat, or plugs made of loose media, little bits of material will not break off and clog up irrigation.

Pro: Easy to handle

Polymer-bound plugs are separate from each other, and one piece. This means you won’t have to cut or tear plugs apart, avoiding root damage and saving time.

Handling polymer bound plugs won’t create dense portions in the root zone either, which happens with some media types and causes damage and anaerobic zones.

*This does not mean that you should be handling seedlings a lot! The less they are handled, the fewer disease opportunities there are!

Pro: convenience

Another trait desired by many growers in tightly spaced systems with a need for labor efficiency is that polymer bound plugs can be extremely convenient. Many are shipped already in the seedling tray, and already damp; growers simply pull the tray from the package, adds seeds, and place it into their seedling system.

This saves time sorting plugs (or loose media), filling trays and wetting the material.

Pro: faster germination

Compared to some other types of media, polymer-bound plugs have shown faster germination.

Even a few days can speed up your growing cycle. If you save 2–3 days every turn, you can fit in one extra harvest a year; that translates into more revenue! This could very well make polymer plugs worth the investment, depending on what your current seedling operation is.

Con: hard to reuse

Polymer-bound plugs can’t be reused in the typical sense. Once seeds grow in a plug and are transplanted into the maturing system, roots tend to take over. By the time you harvest the crop and take the root ball out of the system, the plug is too overgrown or torn apart by roots to be reusable.

Growers can reuse plugs in which the seed did not germinate, but this can open doors for disease and is more labor intensive. The final option is composting. Polymer-bound plugs will decompose and compost over time, but it takes 3–4 years for them to break down.

Balancing benefits with cost

Of course, an important factor in choosing the substrate is weighing the benefits gained against the cost. In a comparison between polymer plugs and soil plugs for a small grower, for example, polymer plugs will come in second for initial and recurring costs.

Something that growers must calculate into that is how long it takes them to plant each tray of plugs. If it takes growers 2 minutes longer to plant a tray with soil, that might make up the price distance between the two types.

So which is better?

We’ve used polymer-bound plugs, soil, rockwool, peat, oasis, and more. None is universally best, and growers have many options. But polymer bound plugs are the best for growers who want to run a clean hydroponic system and reduce labor time.

Want more info on hydroponic substrates?

There’s a whole course on substrates (media) and how to choose the best one for your system. Check it out here:

The benefits of polymer-bound seedling plugs have caused a rise in popularity, especially in systems with recirculating irrigation & limited space.

Seed Starting for Indoor Farmers: Types of Seed Plugs

Classic seed plugs might not be your best option

You’ve probably seen the video we did a while back on the seed starting and transplanting process that we use in our greenhouse. In our greenhouse we use traditional soil, peat, coco coir, etc. But many of you are concerned with solids build up in your indoor systems. This is a real concern, especially if your don’t have really aggressive aeration! And since we think that indoor systems should be as simple and low-maintenance as possible, we’re going to show you a few seed starting options for indoor growers.

(For more videos on labor-smart indoor systems, check out this Youtube playlist.)

Non-messy alternatives to potting mixes

1) Flexible plugs.

The most popular alternative is flexible plugs. Flexiplugs is the brand that Dr. Nate is showing off in the video; this is a coir or peat product that is bound together with a polymer. Slow-release fertilizers can be incorporated into flexible plugs, which can eliminate the need for hydroponic nutrients in your seed starting system. (Which is really handy, in our opinion.)

Flexible plugs compress nicely when being planted between the Matrix Media in towers.

The major downside of flexible plugs is that they are rarely as compostable as peat or soil.

They can also be pricey if you’re buying them in small quantities. (Several hundred to several thousand). If you’re buying flexible plugs on a large scale (tens of thousands or more), then flexible plugs could be a very easy option for you.

2) Rock wool.

Another option is rock wool. (You might be familiar with rock wool as insulation, which is another use for it.) Rock wool is very handy for seed starting, and especially for cloning plants.

Personally, we aren’t crazy about rock wool here at Bright Agrotech because it is not bio-degradable; once it gets sent to a landfill, it’s going to be there forever. We recommend something more environment-friendly.

3) Other products.

Jiffy Pellets: Jiffy Pellets are another option. They can be pricey and harder to handle, but we’re keeping them in minds as a great option for automation in the future.

Oasis products: In our experience, Oasis products tend to smash up and leave bits of plastic in the system. We don’t typically recommend Oasis products for this reason.

Bare root plants: You can absolutely use bare root plants, especially if you’re using wicking strips. The most common problem with bare root plants is that the roots of the seedlings get damaged during transplant and the resulting plant shock slows down your production cycle.

Because of this, we usually recommend using a growing medium for plant products.

In conclusion

Today we talked about types of growing medium for seedlings, including flexible

My final recommendation is to start with a flexible plug because they are the easiest plug to use. Once you get comfortable with growing, transition to a loose growing medium that composts easily and that is renewable.

Did I miss anything?

Do me a favor and tell me what YOU use to grow seedlings.

Look forward to more videos on the planting products and on indoor growing. Keep from missing out by following us on Youtube.

Seed starting doesn’t have to be messy. For indoor growers, clean plugs keep the system from gunking up. Learn about seed plug options for indoor growers.