Jiffy peat pellets have been around for decades and offer an alternative to soil blocks and cell packs for growing seedling transplants. Their small circular shape lends itself well to transplanting into small spaces such as gutter and PVC gardens, and the roots grow right through the pellet which helps lessens transplant shock. They've been working out really well with good seed germination and good transplanting success.
Jiffy peat pellets are small discs of compressed peat encased in a thin perforated net. They come in many sizes but the ones most often used in the pre-packaged kits are the #6 (36mm) and #7 (42mm) sizes. The windowsill kits use the smaller #6 peat pellets and the larger kits use the #7 peat pellets. All you need to do is add water to the pellets in the tray, wait for the pellets to hydrate, add seeds, then replace the cover to keep the humidity level up until the seeds sprout.
Once the seedling is large enough, the entire pellet can be transplanted into the soil eliminating root damage and transplant shock, because the roots grow right through the perforated netting. It's not easy to remove the netting without the peat pot crumbling so it's just easier to transplant the whole pellet. The photo below shows lettuce roots in a gutter garden after just three weeks.
There are many Jiffy kits available at Amazon and other retailers, but for us the 12-pack windowsill kit works best because the Jiffy pellets are used mostly for gutter garden lettuce, and the smaller pellet size fits perfectly inside the narrow gutter. We've tried the 25-pack Jiffy kit as well as the 24-pack windowsill greenhouse (both shown below), but the small windowsill kit still works best. All the Jiffy pellet kits come with a tray, peat pellets, and a cover. As you can see in the photo below, the Jiffy trays fit into the standard 10/20 seedling trays which is helpful for shuttling the transplants back and forth to the coldframe in the Spring. You can also buy replacement pellets which makes the whole setup very inexpensive and reusable. Just check that you're getting the correct size pellets.
The photo below shows our first crop of lettuce using the Jiffy peat pellets - six Deer Tongue and six Mirlo.
The peat pellets have a 100% seed germination and plant survival rate so far. I'm not sure how well the Jiffy pellets would work for larger plants like eggplants or cucumbers, but for lettuce they work great.
Here's a video showing the Jiffy pellets being used for growing gutter garden lettuces: