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.)

polymer-bound seedling plugs

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.

Read more: What to look for in a growing medium 

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.

polymer-bound seedling plugs

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.

polymer-bound seedling plugs

*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.

(Read about how to build a space-efficient seedling system here.)

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.

polymer-bound seedling plugs

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. 

Read more: Tips on choosing a substrate

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:

Hydroponic Substrate Selection

Take the course.

Upstart University

Upstart University