Best Indoor Seed Starting Container Choices to Get Growing This Spring

Spring seed starting season is among the most exciting and eventful times in the life cycle of the gardener. March and April are a great time in much of the US (and other temperate regions of the Northern Hemisphere) to begin getting seeds started indoors under lights or in a greenhouse. In just a few short weeks, seeds started now will be ready to transplant into the garden!

Right now, Greenhouse Megastore is in a flurry of action getting spring orders out the door. It’s an already busy time made more challenging this year by the outbreak of COVID-19. Many people are picking up gardening again, or for the first time, and many customers shopping on the site have questions about how to start their seeds and in which kind of trays or pots to start them. Understandably, there are a lot of options and this can get confusing really quickly. There are myriad different ways to start seeds indoors and which style you use will vary, depend on your garden style, crop needs, your experience, and other factors.

One often overlooked consideration when starting seeds is container choice. Today, I’ll discuss the range of container choices available to you so that you can decide which one fits your needs, budget, and style of growing.

Plastic Trays

Plastic is far and away the most popular option for seed starting. Historically, growers would build their own flats out of wood. They could use them for seed starting or as a carrying tray to move pots. As the industry developed, the plastic revolution brought more options and tools to the industry. The plastic flat was developed because it allowed a grower to cheaply replace missing, loaned, and broken flats. With the advent of the plastic flat, growers no longer needed to spend time in the winter months building costly replacement flats, and plastic can be cleaned and sanitized easily.

If you are considering using plastic trays and flats they need to be stored after use and cleaned before they can be reused. Growers need to be careful when using plastic as seedlings can become root-bound if left in a cell size not suited to the crop. There can be some transplant shock when plants are removed from the cells too. Plastic trays and flats will eventually wear out and need to be replaced.

1020 Traditional Trays & Flats

The natural place to start when discussing seed starting is with 1020 trays, sometimes called flats. Flats are the unsung heroes of the horticulture industry. They are thin, affordable plastic and can be reused for several years. Some come with holes to allow for drainage, and some are solid.

Flats are the unsung heroes of the horticulture industry.

As the industry grew, there was a movement toward standardized sizes of plastic tray that helped economize shipping and production. Eventually, most of the industry settled upon the use of 1020 flat as the standard. This standardization also allowed for development of other product design which supported the flat such as benching, carts, lighting, irrigating, and heating options.

The average life span of a 1020 flat is about 4 years. These thin flats sometimes don’t carry well and can be prone to cracking, so in response the industry developed a carrying try, often referred too as web flats. These help increase the longevity of the 1020 and save the grower time and money in the spring by doing away with the need to wash and sanitize or replace heavier more expensive trays.

Tray or Flat Inserts

With the standardization of the flat came the production of the celled “insert”. An insert is just what it sounds like: a plastic piece with different cell configurations that can be dropped into a standard flat to make the open flat have uniform cells. This cellular structure allows for uniforms seeding and easy transplanting.

The 1020 tray or flat with a celled insert of some kind is our most popular seed starting setup.

Inserts are made from very thin plastic, and are generally discarded every year. They are very cost effective and allow the grower to save the tray or flat itself every year, but get new inserts every year, possibly in different configurations. This allows growers to save money versus buying more expensive celled trays every single year. The 1020 tray or flat with a celled insert of some kind is our most popular seed starting setup.

Propagation Trays or Plug Flats

Propagation trays are more geared toward larger scale growing operations and the automation that comes with them, but any size gardener can use them. Their innovation derives from the open flat used for seed starting and the addition of cells to them. Cells are individualized compartments in the tray and help develop stronger roots in transplants by confining the roots and encouraging branching. The flat’s cell size will vary depending on the length of time the seedling will be in the tray, the crop’s canopy size, space available in the greenhouse, and other production considerations. The larger the plant or the longer it will be in the cell, the larger the cell size needs to be.

The larger the plant or the longer it will be in the cell, the larger the cell size needs to be.

The most common cell sizes for transplants are 50’s, 72’s, 128’s and 200’s. The cells in these flats are square and very closely spaced. In general, the square shape used in greenhouse containers maximizes the number of plants you can fit in your greenhouse and aids more efficient tending to the crop. For larger crops, such as melons, pumpkins and cucumbers there are cell flats with 38 or 24 cells per flat. The cells are larger in depth and width, allowing for larger root balls. These cells are rounded and spaced farther apart to allow for greater canopy development of the plants. Round containers also help save the grower on planting media, as they will hold 25% less media, offering further savings to the grower.

The tiniest of cell offerings, the 128’s and 200’s require the crops to be “bumped up” or “potted up” into cell flats when their first true leaves appear. They can also be used for lettuce, carrots, and onions which will be put directly out into the garden. The thin walls and many cells on these l flats are advantageous because they conserve seedling heat mat or germination chamber space. Finally, when you see a star shaped pot or cell, that is designed for crops with larger tap roots or for when maximum root development is needed, generally long-term storage and perennial crops are started with the star shaped cells.

Both 1020 flats and plug flats have a lip which will allow for the use of a clear dome to be used to increase the humidity around the seeds.

Biodegradable Pots

Variety of plants in containers

Plastic is, admittedly, not the most environmentally friendly material. For those who do not want to use plastic, there are many types of biodegradable pots. Biodegradable pots can be planted directly into the ground when transplanting and will break down in the soil. This makes them a very easy and sustainable option for growers of all sizes.

Since they can be planted directly, biodegradable pots help reduce transplant shock often seen with plastic trays. They are super-easy to plant because the plants don’t have to be removed from the pot. Best of all, they also don’t leave behind any hard-to-recycle plastic waste! This makes them popular with home gardeners. The only disadvantage of biodegradable is that they need to be replenished each season. They do require carrying tray as they can break down before ready to be transplanted. For growers tight on space, and they are not as space-efficient in the greenhouse as plastic.

Cow Pots

Our favorite for organic growers is the CowPot™. CowPots are OMRI-listed and appropriate for certified-organic operations. CowPots are made from 100% composted cow manure, which adds some inherent fertilizer to the soil as it breaks down after transplanting and stimulates root growth. These are individual containers that can be planted along with the seedlings. The pot then breaks down in the soil, allowing roots, water, and nutrients to pass through it. These biodegradable pots are similar in size and shape as the inserts or sheets.

Jiffy Pots

Jiffy pots are another very popular biodegradable pot solution. Jiffy pots are made from compressed peat moss, the very same material most of our most popular growing mediums are derived from. The peat is compressed into a sheet and then formed into a pot. These pots are then treated the same as any other pot, with the exception they can be planted directly in the ground.

Coir Pots

Coir pots are very similar to peat pots except they are made from coconut husks. This makes them a bit more sustainable that peat pots, since coir is a renewable resource and is often produced as a waste material in the coconut industry.

Soil Blocks

For the ultra-earth friendly grower who doesn’t’ want to buy supplies every year there are Soil Blockers. These handy tools make cubes from potting media. Your seedings will be growing in a free-standing cube, rather than in some type of container holding the medium.

Soil blocks can produce excellent transplants, thanks to the “air pruning” that occurs when the tips of the seedling roots grow to the edges of the block and dehydrate, triggering growth of secondary roots which then spread until they reach the edges. In other words, no root binding! The problem with soil blocking is that they take a great deal of time to make and the media mixture has to be just right. For those growers with the will to go down the soil block route, it is often touted as the very best organic growing method. The master of large scale market farming and a pioneer in the industry, Eliot Coleman, recommends soil blocks himself and has worked closely with Johnny Seeds to develop both a blocking tool and soil mix recipe.


There are many options available to you the get your seeds started and ready for spring. If you’re just getting started, maybe the ease and affordability of plastic is best for you. If you are more sustainable focused then one of the biodegradable options might better suit. Either way, there is a container option for you and chances are great that Greenhouse Megastore can help you get set up today!

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