For a demonstration, I purchased three different sizes of wildflower seed mixes from local garden centers. The flowers that result after sowing are very similar in the different mixes so for our purposes they're the same blend. But even for seed mixes that are alike, packet quantity and prices can vary dramatically.
Closer analysis shows less of a difference. On the back of the seed packets or bags you'll find a label that lists the specific seeds contained and the composition of the mix. While the 640 gram bag lists an impressive array of flowers, it also shows that 95.89% of the mix is inert matter, walnut hulls. Only 4.11% of the seed mix is actually seeds. The actual flower seed weight is 30 grams. That makes the cost for those seeds about 33 cents per gram.
The 113 gram packet reveals that there is 81.25% inert material and only 21 grams of actual seed in the box. That makes the cost of those seeds about 24 cents per gram. The middle size packet is about one-fourth as expensive as the other two.
For the most part it comes down to how we gardeners sow our seeds. The seed distributors have designed the seed packets to match the needs of the consumer. A problem arises in that many of the consumer gardeners don't know what their needs really are.
I'm here to help. Let's start by comparing the two larger sizes, with inert matter in the mixes. The middle size, the one that is obviously the best relative price, says that it will "plant" 50 to 75 square feet. The largest bag, the most expensive, says it will "cover" up to 1000 square feet. What?!
The largest bag has just 50 percent more seed but will cover up to 2000 percent more space?! At first glance that doesn't make much sense. But that's where the walnut hulls come into play. The bits of walnut act as little spacers to make sowing more effective. Believe it or not, fewer seeds can actually cover a larger area when you add fillers to the mix.
Have you sown individual seeds in a furrow or over open soil? Ever notice that the seeds tend to clump together? You get a couple seeds side by side in one spot, maybe three or four close together some place else, and unevenly spaced seeds everywhere. It's usually not a big deal because we know that thinning out the little seedlings is part of gardening. After the seeds sprout we go back and pluck out the seedlings that are too close to others so that they are more evenly spaced. Sure, it's a waste of seed, but that's the way gardeners do it.
When you have inert matter in a seed mix and sow the same way, now there's a better chance that when the little bits hit the soil and they clump together a part of the mass will be seed and part will be the inert matter. The higher the percentage of inert matter, the higher the possibility that individual seeds will be separate from each other. The middle-size seed packet has fewer seeds and less filler than the big bag, but when you sow the seed mix it will separate in a similar way.
A visual inspection of the mixes shows a difference in the size of the inert matter. The big bag, with larger distribution capability, has much larger chunks of walnut hulls than the middle box which appears to have little pieces of corn cob. Similar size piles of the mixes shows a distribution of many more seeds when the inert matter is small. The bag with big chunks of walnut hulls actually had comparatively few seeds in a pile.
Let's get back to the small packet that is 100 percent seed. The distribution of the seed is only as good as your fingers can control. Theoretically, the seed can cover up to 250 square feet; it's about a quarter of the actual seed count of the big bag that can do 1000 square feet. Try as I might, I don't think I can take a handful of small seeds and evenly spread it over such a large zone.
I might be able to get closer to 25 square feet; the pure seed is about one-third the amount as the middle-size mix and that's one-third the coverage to "plant" the 75 square feet on the label. Looking at the number of seeds in the packet, I think it's a pretty good estimate of the seed coverage. However, I'd be lucky to not dump all the seed in one clump on the soil.
The middle mix will "plant" up to 75 square feet. Again you'll get even distribution of separated flowers, but you can expect that they'll grow closer together. This may be what you want if you're looking for a space to be filled by plants.
The small packet of pure seed gives you the greatest control over how thick your planting becomes. There is no guidance on the label for expected coverage, it's up to you. If you want well-spaced plants you can try and broadcast the seed over a large area. If you want plants close to each other you can sow in a small space and thin seedlings as needed.
Three sizes of seed mix means three different ways to sow and three different outcomes of flower spacing. Comparing these three packets, you pay more for the privilege of covering a large area or to have control over sowing a smaller area. The best price, the middle size, may actually be the best deal also. For a gardener looking to sow an average size bed of 50 to 75 square feet, that mix should provide even seed distribution and effectively fill the bed with plants.
For a smaller bed it makes sense to buy the smaller packet and try to get even distribution by hand. For a very large area it makes sense to pay more for the ability to evenly distribute the seed.
It comes down to you and what you want to do with the seed. Before you make your seed purchase based on cost alone, take a look at labels and compare them. Find the seed packet that most closely matches how you sow and your desired outcome. In the long run you'll probably save yourself time, effort, and money.