In this last of a series of three postings, I describe the pros and cons of various seed starting containers and make my recommendations.
Clay Pots - In my opinion there really aren't many "pros" to using clay pots. Their heavy, break easily, and are hard to clean at the end of the season. All hard sided pots, including clay ones can also cause seedlings to become root bound. Root growth is stopped by the sides of the pot and the roots respond by creating a circular root mass along the pot walls. Gardeners need to tease out the roots from the root ball prior to planting to prevent the seedling from becoming stunted. This is not a big deal, but its a disadvantage of using hard sided pots. On the plus side, clay pots can be re-used each year, but they also need to be thoroughly cleaned and sanitized. As I mentioned in my prior posting, it's best to plant seedlings in a growing medium that does not contain any micro-organisms that might kill the delicate seedlings. To ensure that a clay pot is free of such nasties, you need to clean the pot and then dip it in a dilute solution of water and bleach (10% bleach + 90% water) for ten minutes. Not fun and not very ec0-friendly due to the bleach. For these reasons, I don't recommend clay pots.
Individual Plastic Pots - small plastic pots are light, durable, and are not porous to the air so they retain water well for the seedlings planted in them. Plastic pots are re-usable so you can get many seasons out of them before having to purchase new ones. Like clay pots, they need to be cleaned and sanitized at the end of the season to prevent spreading disease and also like clay pots, plants can get root bound if allowed to grow too long in these pots. That said, I think plastic pots are are still one of the best choices available to gardeners. Recommended highly.
Multi-Cell Plug Trays - these are thin-walled plastic containers like the ones that plants are sold in at the garden center. They are inexpensive and retain water well. However, they are not particularly durable and don't lend themselves well to re-use in my opinion. I think many people end up throwing these in the garbage at the end of each year so they make for lots of landfill. Additionally, I sometimes find it difficult to get plants out of these trays, particularly if the plants are young and have not developed very vigorous root systems to hold the soil within the cell. These are an "OK" choice in my opinion, but I think it's easier to handle individual plastic pots than these multi-cell containers.
Complete Seed Starting Systems - I used a system sold by Burpee a few years ago called the Ultimate Seed Starting System. The system includes a bottom tray that acts as a water reservoir, a mat that wicks water upward from the reservoir, a series of multi-cell trays for planting seeds in soilless mix, and a dome to cover the seeds and keep them moist. I found this system to be incredibly easy to use and did an awesome job of keeping the seeds at just the right level of humidity. They can be re-used in theory, but I found that in practice that I ended up destroying the containers when attempting to remove seedlings from the little cells (see below photo). In my opinion, the flimsiness of the plastic is the Achilles heel of the product. If you use this type of system, you probably need to buy a new one each year (I believe they cost about $20) However, the system worked so well as a seed starting product that I still would recommend them to others. You just need to decide how bad you feel about tossing the plastic in the garbage each year. If this doesn't bother you, I would place containers like the Ultimate Seed Starter kit at the top of the recommended list.
Biodegradable Pots - these products are generally made of peat moss that is pressed into a the shape of a pot. Jiffy pellets are similar but are both pot and soilless mix in one. These types of containers are lightweight, relatively cheap, and easy to use. In theory the main benefit of these products is that they can be planted directly in the ground - no pot to worry about cleaning the following year and no having to remove the seedling from the container and its attendant inconveniences. Sounds good, but in practice many gardeners have had problems with peat pots. Specifically, the wall of the pot does not degrade quickly enough after transplanting, and the plant becomes root bound and stunted. Peat pots can be purchased for around $.10-$.20 each so they are not that expensive. I find the price, convenience and biodegradable nature of these products compelling, so here's what I'd recommend; if you want to use peat pots, tear away the pot walls entirely before transplanting the seedlings into the ground. That way your plant wont become root bound. Then throw the pot itself into your compost bin. No pots to wash, no bleach, no plastic in the landfill. Recommended, but only if your remove the walls of the pot prior to planting.
Wooden Seed Boxes - These are generally made of a natural rot resistant type of wood such as redwood or cedar. This type of container is not very common. You'll need to either build it yourself or buy it from a specialty garden retailer. They are very eco-friendly and reusable, but they are also heavy, expensive, and not really cleanable. These boxes don't have individual compartments for the seeds so, you need to tease the seedlings apart when removing them from the container. I made some of these last year to give them a try. I wasn't happy with the results and won't be using them this year. Not recommended.
Soil Blocks - I'm going to try these for the first time this year. A soil block is a pot-less container made only from soil. You purchase a soil block maker and then make the blocks yourself using your own soil mix. The soil block maker looks and operates like a cookie cutter. You mix up a batch of potting soil and then push the block maker into the soil. You then push down on a release mechanism and the block maker pushes out perfectly formed cubes of soil into which you plant seeds. When you are ready to transplant, you just place the soil block in the ground. Soil block makers seem to have many advantages. There are no pots to clean or store, no plastic to throw in the garbage, and unlike hard sided pots, plants in plant blocks won't become root bound because when the root of the seedling reach the outer edge of the seed block, they responds by suspending growth, rather than circling the walls of the container. Another advantage of soil block makers is that they are claimed to make it easy to "pot up" the seedlings -- In other words, you can start the seedlings in tiny soil blocks and then transfer the tiny block into the top of a larger soil block if the plant gets too large for the original block. This is made easy by the fact that larger sized soil block makers are designed with pegs that create an indentation in the top of the soil block into which a smaller block will fit perfectly. You just pop the smaller block into the larger one (see below) I purchased a set of soil block makers and will be giving them a try this year. One thing worth mentioning is that soil block makers use a totally different potting medium than any of the containers previously mentioned. Specifically, the medium needs to be much heaveier than soilless mix in order for the soil block to hold its shape. Online retailers who sell soil block makers also sell soil mixes for these devices, but I think most people will want to mix their own soil rather than pay the shipping cost on a heavy bad of soil. To make your own soil block mix, you'll need to buy peat moss, vermiculite, garden soil and compost. And there's the rub...the garden soil and compost can harbor some of the nasty micro-organisms that can kill seedlings, so the one potential disadvantage of soil blocks is a greater risk of killing the seedlings due to fungal or other infections. That said, I'm going to give them a try this year and see what happens.