I agree with Beansie, read up on what each of the elements do for plants and where they are best used.
Nitrogen encourages growth of foliage and green color; phosphurus brings forth roots and flower.
Potassium (potash) helps plants have good health, strong stems.

For flowering plants, after their initial growth, a 15/30/15 fertilizer is welcomed by the plants.
Miracle Gro is one such as this but there are as many on the store shelves as there is stars in the sky.....
well, not as much as that....but plenty.

A note about bone meal. Critters love it and will dig, dig, dig to find the bone. Dogs, skunks, cats, coyotes.

Bump. I know it seems like it would be easy to find, but my local nursery didn't have anything appropriate, nor did Home Depot, last year. I've been looking on Amazon, and Bonzai fertilizer is the closest I can find to 5-10-10 or 10-20-20. Other than the ratio, are there types of ingredients that are important?

My related question is that I planted 20 rhizomes in my new yard last year, and only one did well. The "soil" is really clay, except where it's been sort of amended, and even that area didn't nurture them very well. I have a back injury and really can't dig them back up and re-amend them...I dug down 12 inches, the first time. Would using better fertilizer do the trick, or should I just give up?

I see you are in zone 10 - it may be that your Dahlias didn't do well because of the summer heat. I don't know what time of year you put them in, but they may do better this year since they will come up early. I know of someone in FLA who gets an early spring "crop", then cuts them down to the ground in July, and they grow back for a fall "crop".

I have only ever fertilized mine with bone meal and well composted manure. If you mulch well with compost every year, the worms will eventually help mix it down into the soil for you.

Like all plants, dahlias need sunlight, water and fertilizer. When somebody says they have difficulty in growing a plant each of those three ingredients must be correct. Assuming you get the tubers to sprout and get above the soil level, make sure they stay well watered(dahlias need more water than most plants), make sure they are in full sunlight(they hate full shade or even half shade) and that they have some fertilizer. Just about any fertilizer will work, a little bit at a time. Too much is bad, too little is probably not likely if you fertilize them every couple of weeks with no more than a level tablespoon sprinkled at the base of the plant. If you are organic, you would use manure or bat guano, or blood meal, or leaf mold, or very rich compost before you plant and hope that there are enough nutrients for the plants for the entire season. If you are rich, you can use Osmocote time release fertilizer and give them about 4 tablespoons per plant when you plant them and maybe one more tablespoon on August 1st. Once the plants are full sized(most reach 4 feet) stop fertilizing as they have enough to last the rest of the season. Notice I did not say the formula for the fertilizer. That is much less important than the small doses of regular feeding. And you could use Miracle Grow once a week too, just make sure it is a weak solution. Fertilizer is plant food and just like human food you do not eat one meal each month but you eat every day. Plants too need small meals on a regular basis.

This post was edited by teddahlia on Tue, Feb 18, 14 at 12:38

Thanks for your responses; I am inland in Zone 10. My dahlias were amazing by the coast (Z 10 also), so maybe it is too hot for them, here; they were planted in April, and I've left them out, in hopes that their roots might develop better, the second time around. They're in full sun; but the only area I had to use (and I'm the opposite of rich, right now, due to the back injury, which prevents me from either hiring someone to fix this problem, or doing it myself) is like half gravel, half amended soil. The gravel is dark, so might be overheating the roots, in addition to just being in the way. I did do a foot deep hole with real soil and compost when planting, though. Should I perhaps mulch them to try to keep the roots cool?

Most of them were stringy and short, and produced about two flowers, each; the two were nice, though. Only one got full sized, and it was about 4 ft. In my old garden, it would probably have been bigger and fuller.

They got a lot of water, but not much fertilizer, as I'd read that 0 Nitrogen was imperative, and I couldn't find such a fertilizer. I've recently read that the 0-10-10 is used for hydroponics, and has something to do with root development....Nitrogen early for root development, then none, using the 0-10-10 for the rest of the season, I think. Does that sound right? Since my poor dahlias are having to contend with such horrible "soil", maybe the nitrogen in the beginning would be a good idea. One blog recommended using a blast of Nitrogen to get the roots going and another, later, just as the blooms are developing, to make them bigger...but that was for Medical MJ, I'm sure. I don't know if it applies to normal flowers.

If I can't figure it out, I'll do as you suggest and feed lightly and often with something else. The Osmocote is cheap on Amazon; is there something special about it?

Should I perhaps mulch them to try to keep the roots cool? Yes, I have heard of excellent results using mulch in hot weather areas. Pay no attention to the myth that dahlias do not need nitrogen. All plants need nitrogen and without nitrogen dahlias grow very poorly. The myth goes back to when people were over fertilizing dahlias in the Fall and too much nitrogen in the Fall does affect tuber storage but does not affect the quality of the blooms. In my fertilizer recommendation above, you will note that the fertilizer is drastically reduced in early Fall. Any fertilizer that you use must have nitrogen. As I said just about any formula will do as long as there is some of all three nutrients. They are abbreviated as NPK and the first number is nitrogen the second phosphorus and the third potassium. Having all three gives the plants a balanced diet. Osmocote is always formulated with enough nitrogen and all the major elements and as a bonus it includes the minor elements needed for plant growth. It cost about $100- for 50 pounds. Farmers do not use it because their fertilizer costs about $30- for 50 pounds. The advantage of Osmocote is that you apply it just once and the wax encapsulated pellets release fertilizer evenly for about 2 to 3 months. Farmers using regular fertilizer must fertilize several times during the season with the cheaper fertilizer. I have used both and If I were rich I would use Osmocote. I use about 200 to 300 pounds of fertilizer each year. There are other commercial formulations of time release products that can be special ordered by the ton. The cost of those is about 30% more than regular fertilizer but are not available to the casual gardener. And this cheaper version of time release only releases the nitrogen over an extended period and the other nutrients release immediately. Osmocote is much better.

Great, thanks! Mulch soon, fertilize lightly often, not in the fall.

You are a master of summarizing in a few words.

It is interesting to note that there are so many opinions, it is hard to know which one is correct. According to the experts, "Dahlias require a low nitrogen fertilizer, such as used for vegetables. We recommend high percentage potassium and phosphorus fertilizers such as a 5-10-10, 10-20-20, or 0-20-20. First applications should be within 30 days of planting and repeated again approx. 3-4 weeks later. One of the biggest mistakes made with dahlias is over feeding them. Avoid high nitrogen compost and high nitrogen water soluble types as they promote weak stems, small blooms, or no blooms, and tubers that rot or shrivel in storage..

I'd like share that although fish emulsion is nitrogen dominant, at 5% and requiring microbial digestion to release it does not represent a substantial dose of N. Synthetic fertilizers comprised of nitrate, or ammoniacal nitrogen represent a far more potent dose of N due to their solubility. My approach is to rely upon a 1:1 ratio organic fertilizer such as Jobe's Organic granular 4-4-4 supplemented at the time of planting by incorporating Jobe's Organic Bulb granular beneath the tuber. I achieve consistent growth and very nice bloom count without needlessly dumping excess phosphorus or potassium into the ground. This season I began using fish bone meal beneath the tuber and am anxious to see what the outcome will reveal. Our season here in the Pacific NW is cooler well after the lower latitudes have been experiencing high temperatures.

"It is interesting to note that there are so many opinions, it is hard to know which one is correct. "

Part of that problem comes from well-intended generalizations when specific information about gardening soil would change the recommendations. There's a lot of 'conventional wisdom' information still out there, too, like feeding no nitrogen early in the season. Moderation is the key.

One of the most important things about fertilizing is to make sure the pH is as close to 6.2-6.5 as possible, which allows your dahlia plants to take up all that fertilizer you're throwing at them.

But as experts go, I'd follow Teddahlia's advice in the above comments any day. As a master grower who has hybridized for decades, has done his homework many times over and always gives well-reasoned explanations with practical advice, he has earned my attention.