We Tried Home Depot's Organic Seed Starting Mix And DIYed A Winning Recipe

Anyone who has seen how easily the weeds and dandelions appear in their garden has probably looked at pricey seed-starting mixes in puzzlement. After all, seeds don't generally require much to turn into seedlings other than a little warmth and moisture right? So, one naturally wonders, why then do you need specialized little bags of fake dirt to plant seeds in, when your kindergartner can sprout them in wet paper towels? There are some answers, of course.

Seeds' roots need structure to cling to, and the availability of both air and moisture needs to be consistent. Moreover, the medium needs to be fairly clean, though sterilized seed starting mixes might be a bit over the top. And so we found ourselves questioning the effectiveness of the $8 Jiffy Organic Seed Starting Mix (incorrectly called a "kit" on the Home Depot website). It is made up of peat, coconut coir, and vermiculite, all lightweight materials that are mainly known for holding a lot of water. Since we already had the first two of these ingredients on hand, and use them regularly to start seeds, we wanted to find out if there was any point in buying this little bag that seemed inordinately expensive.

Jiffy is a line of seed-starting products, mostly known for producing tray inserts that let you start dozens of seeds at once. Having long since transitioned from the expensive cell trays to just about anything that will hold dirt — mostly cast-off trays from garden centers — we had some Jiffy cells sitting around. We promptly filled them with a half-tray each of SunShine sphagnum peat moss ($18.97 for about 64 lbs., or $.30/lb.), Best Coco Peat ($27.95 for an 11-lb. compressed block, or $2.54/lb.), the Jiffy seed-starting mix ($7.97 for 3.4 lbs., or $2.34/lb.), and a 50/50 mix of peat and coir ($1.42/lb.).

We started two radishes and two lettuce seeds per cell, but didn't really expect to see any difference in germination, so we also came up with an experiment to evaluate each of the four materials based on moisture retention and crumb — the fluffiness of the medium's structure. We filled 10 three-inch nursery pots (2 with peat, 2 with coir, 2 with peat/coir, and 4 with Jiffy mix) and set them in a tub of water to soak up until saturated. Once they were saturated, we set them aside for 24 hours in a warm greenhouse to give them all the same opportunity to lose water through evaporation. Finally, we created a slurry using the Jiffy mix and instructions from Aptus Plant Tech to get an idea of the soil's pH levels.

To test for water retention, we weighed the different media — the peat held the least water (40% by weight), while the coconut coir held the most (74%). The homemade peat/coir mixture did about as expected, retaining 53% of its weight, and the Jiffy seed starting mix held 65%. We noticed that the Jiffy mix lost a lot more volume and was very slightly denser when wet than we'd prefer for seed starting. We also noticed when we opened the Jiffy mix that it seemed unusually fluffy — it felt like we were buying a bag of air. But once measured, it became clear that the coir was the lightest (12.76 grams, versus 16.19 for the Jiffy mix and 20.75 for the peat). 

Since our pH meter hasn't been calibrated in ages, the results may not be totally accurate, but our readings are mostly consistent with what the manufacturers and other reviewers have found. The peat tested at a surprisingly moderate 5.6, the Jiffy mix at 6.2, and the coir at 6.4. By comparison, Canadian sphagnum peat moss is typically pretty acidic, with a pH of 3.0 to 4.5. The ideal pH for a seed-starting mix is usually between 5.5 and 7, and all of the media we tested fell within that range.

We didn't expect to see any difference in germination rate, so when the seeds in peat sprouted more slowly we were surprised. In the time allowed (about four days), everything except the peat had a 100% germination rate; the peat had sprouted around 60%. The most likely explanation is that peat is a little too wet for seed-starting; media that's too wet can slow down or prevent germination altogether. Our 50/50 mix, the coir, and the Jiffy mix all germinated identically and in the same timeframe. Since the seed-starting mix doesn't provide any nutrients, we didn't see any difference in seedling color or growth rate.

The Jiffy mix held more water than peat, but there was very little vermiculite visible in the mix, so the Jiffy mix is probably mostly a combination of peat and coir, like our DIY version. Coir would be our choice based on holding moisture, followed by Jiffy mix, then the peat/coir combo. The only difference in crumb we noticed was that the Jiffy mix shrank quite a lot when wet, losing maybe 5/8-inch at the top. The other media remained suitably fluffy.

Coir is surprisingly expensive compared to peat, but the 50/50 combo came in at about 60% of the Jiffy Mix's price. Given the quantities we use, we'll stick with 50/50 and coir. But the Jiffy mix performed well, so we wouldn't hesitate to use it for a just tray or two of seedlings,