Best Fertilizer For Dahlia: Tips On How To Fertilize Dahlias


By: Bonnie L. Grant, Certified Urban Agriculturist

You don’t need to be a collector to appreciate the many colors and forms of dahlia flowers. These Mexican natives have become garden staples across the nation, providing large easy to grow, inexpensive bushes adorned with blooms all summer. Fertilizing dahlias is key to encouraging flower laden stems and big leafy plants. The best fertilizer for dahlia plants should be high in potassium and phosphorus but low in nitrogen to prevent leafy bushes with few blooms. Start planting early for dahlias that will make your neighbors green with envy.

Dahlia Fertilizer Needs

Healthy tubers are the first step to vigorous dahlia bushes. Choose tubers that are free from blemish, chubby, and have plenty of growth eyes or nodes. Proper soil preparation and knowing how to fertilize dahlias will also help you achieve the massive plants with prolific blooms that are the goal of any dahlia gardener.

Feeding dahlia plants within 30 days of planting will jump start the tubers production of sprouts and stems, while giving the plant enough of the right fuel to enhance flower production and healthy root development.

The “when” and “how” are two great questions regarding feeding dahlia plants, but don’t neglect the “what.” Dahlias are heavy water users and feeders. It takes a lot of energy to stimulate all that massive growth and flower production.

Unlike vegetables and other plants, flowering plants like dahlias need little nitrogen but do need the other two macro-nutrients in higher doses to promote blooms. Choose a formula with the first number in the fertilizer ratio lower than the last two numbers. The first indicates the level of nitrogen, while the second and third numbers indicate the amount of potassium and phosphorus. These crucial nutrients are the energy behind dense upright bushes and branches laden with flowers.

Best Fertilizer for Dahlia

Expert dahlia growers recommend a fertilizer formula with numbers such as 5-10-10, 10-20-20, or even 0-0-10. Note the lower first number, which denotes the amount of nitrogen. Fertilizing dahlias just a couple of times can make a big difference to the flower yield.

Just be careful not to over fertilize, which can cause yellowed leaves, drooping, leggy plants and actually diminish overall plant health. You can balance dahlia fertilizer needs by enriching the soil prior to planting. Till the soil deeply, at least 10 inches (25 cm.), and incorporate a generous amount of finished compost or leaf litter to help with porosity and drainage, as well as increase soil fertility.

How to Fertilize Dahlias

Your plant food will give a recommended amount per square foot for flowering bushes and it is best to follow manufacturer’s instructions. Approximately ½ cup (226.5 gr.) spread around the plant in a 2-foot (61 cm.) ring and lightly worked into the soil should be sufficient. Water in the plant food so it can start to seep to roots for quicker uptake and to avoid burning surface roots.

Apply food 30 days after planting the tubers and again in a month. This will get the plants the jump start they need and additional fertilizer should not be necessary. Remember to water frequently, remove any competitive weeds and watch for pests and disease. Combat issues as they arise for big, bountiful dahlias.

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Read more about Dahlia Flowers


The 5 Best Fertilizers for Dahlias

Only the best fertilizers will deliver results for your dahlias. The problem is that there is a drastic difference between the quality of different products in local gardening shops. It’s difficult to differentiate between good products and bad products because of all the media hype. Don’t make the mistake of choosing an bad product and finding out too late.

Do you want your dahlias to grow up strong and healthy? If so, you should spend the time to find high quality nutrients. Having a product that delivers the correct nutrients in a way that is able to be taken up by the root zone is of utmost importance. I will help you learn what you need to know about finding a product that uses quality ingredients to deliver the proper nutrients at an affordable price.


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Timing

Choosing the right time to plant is the key to a successful dahlia crop. Dahlias prefer warm soil. Plant after all danger of frost (enter your zip code here to find your last frost date) has past and when soils have warmed to at least 55-60 degrees. If the forecast is predicting a prolonged stretch of wet weather, it is best to hold off on planting as tubers are prone to rot in cold/wet soils. In the Northeast, mid-May is typically the earliest we can plant outside. Plant dahlias around the same time you would plant tomatoes in your vegetable garden. If you want earlier flowers, you can start your tubers indoors in pots about a month before planting outdoors.

Site preparation

Dahlias thrive in full sun and should be planted in a location that receives a minimum of 8 hours of sunlight a day. Dahlias planted in an area with less sunlight will grow taller and won’t yield as many blooms. If you live in an area with hot summers, dahlias will appreciate a partly-sunny spot to shield them from the sun during the hottest parts of the day.

Dahlias are heavy feeders and like soil with lots of organic matter. Because tubers are prone to rot, it’s important to make sure that your soil isn’t too heavy and drains freely and easily if not, amend it with peat moss or sand.

Planting Tubers

Single dahlia tubers will grow into a clump and multiply quite substantially over the course of the season, so it is important that you prepare your soil well to accommodate such growth. Start by digging a hole at least 8” wide and 12” deep. We choose to incorporate an organic balanced fertilizer (find our proprietary mix here) at planting time, but depending on the results of your soil test, you may need to amend your soil with more/less nutrients (nitrogen, phosphorous, potassium, etc). Mix approximately 1/4 cup of the fertilizer in with some of the soil you removed before planting tubers horizontally, with the eye(s) facing up, about 4 to 6 inches deep. We space dahlias at 12" apart, with dinnerplate and larger-headed dahlias spaced at least 18-24” apart. All dahlias that we sell will require additional support (tomato cages or bamboo stakes work well).

Watering Dahlias

Once you plant your tubers, wait to water until you see the first shoots emerge. If you are planting outside there usually is enough residual moisture in the soil to get your tubers to start growing. If you are planting in a container or your soil is dry, water well once at planting, then hold off on watering again until shoots emerge. Tubers need a little moisture to being sprouting but too much moisture will make the tuber rot. It is a delicate balance. Remember that different dahlia varieties take different times to sprout. Early-blooming varieties can take as little as 2 weeks to sprout while other varieties (especially dinnerplates) can take as long as 2 months!

Young dahlia plants do not require much water. Once your dahlia plants are established (8-12” tall), water regularly and deeply. A good rule of thumb is that your dahlias should receive 1” of water every week. In the heat of summer, dahlias will require more water especially if it is warm and dry. When they are actively growing and blooming, dahlias will need more water. Dahlias grown in pots will require more frequent, daily watering.

Fertilizing dahlias

We amend our soil with an organic fertilizer mix at planting time. Once dahlias begin to bloom, we foliar feed every other week with a fish and seaweed formula (low nitrogen). Because our fertilizer blend includes slow-release nitrogen, we avoid giving our plants more food high in nitrogen once they begin to bloom.

Topping (Pinching Dahlias)

A regular practice for dahlia growers is to pinch or “top” your dahlia plants to promote lateral growth and a more balanced plant (one that isn’t too top heavy). When your plant is about 12” tall, snip out the growing tip, leaving 3-4 pairs of leaves (counting from the bottom). This will encourage the plant to send up low basal growth that will not only give you more flowers to cut, but also distribute the weight of your plant better so they are less likely to topple in strong winds and rains. Note: supporting your dahlia is paramount. If you are a home gardener, tomato cages or bamboo stakes work well. If you are growing in large amounts, corralling your plants with tomato twine and T-posts spaced every 6-8 feet or using "‘Hortonova’ netting is recommended.

Harvesting Dahlias

The best time to cut dahlias is in the cool of the morning or evening. Using sharp pruners, cut the stem at a 45 degree angle just above a leaf node. (A good stem length is from the tip of your finger to the crook of your elbow). The plant will branch just below the cut and produce additional stems. Harvest dahlias when the flower is nearly or fully open. Closed buds won't open after the stem is cut. Strip any foliage that will be below the waterline in a vase. Place the stems in fresh water and add flower food as desired. Replace the water and recut stems daily for a vase life of 3-5 days (vase life of dahlias varies depending on variety and size).

Harvest or deadhead dahlia plants regularly to promote additional flowering. Once flowers go to seed, the plants will slow down flower production.

Digging dahlias

In the Northeast, our winter temperatures are just too cold, and dahlias’ thin-skinned tubers will freeze if left in the ground over winter. Tubers can be pulled each fall, cleaned, and stored in a cool, dark room until next spring (see “Storing Dahlias” below). After a hard frost has killed the plant, cut the dahlias down close to the base, leaving about 6” of stem as a handle to help you pull the tuber clump up. We have better success storing our tubers over winter when we let our tubers “cure” in the soil for at least 10-14 days after a hard frost before digging. Waiting is not necessary, however, and tubers can be lifted the same day they are cut down. You should do what works for you and your schedule!

Starting at least 6” away from the heart of the plant, use a digging fork to gently lift your tuber clump out of the ground being careful not to break the tuber’s necks. We find that you will need to “dig” 2-3 times around the plant to get the whole clump out intact.

Storing dahlias

It's important to keep tubers clean and maintain a balanced level of humidity in storage over the winter. Tubers should be kept in a cool (40-45 degree), dark place with 85-90% humidity. They are fleshy and water-filled and cannot freeze. Depending on your location and winter weather condition. If you’re planning on storing your tubers in a place that is dry you may need to store tubers in airtight containers with a packing medium (pet shavings, vermiculite, etc). If your storage space is wetter or you live in a rainy climate, you may find more success leaving your tubers in an open box with no packing medium. You will have to check on your tubers frequently. Tubers that have properly hardened off will maintain their firmness in storage and are neither shriveled (storage location too dry) or moldy (storage location too wet). Know that there is no “right” way to store tubers. The key is to maintain temperature and humidity.


Dahlia fertilizer

What do you use to fertilize your dahlias? I keep reading that it has to be really low in nitrogen but I have never seen a fertilizer like that. I use compost and sometimes bone meal but I wonder if there is anything else I should add.

Candy I use nothing. Haven't ever since a guy who sells them and shows them told me it was actualy bad for them especially the nitrogen which applied to late in the season causes the tubers to spoil in storage. When are you coming. Ernie

Awe, what a great photo, Ernie!

What a sweet picture Ernie. Was that on your property?

You don't need to fertilize - I have seen your soil. All you need to do is toss stuff on the ground and jump back. LOL I guess I won't worry about fertilizing then. The compost and bone meal should be enough.

I am still planning on coming out in July. I haven't set a date yet. Can't leave until I get everything done here and I feel so far behind. It has finally warmed up enough to start putting things in the ground, like tomatoes. So hopefully this week I will accomplish a lot of planting and begin to see the light at the end of the tunnel.

I planted my pots ones in MIracle Grow plus potting soil. Do you think this will have too much fertilizer for them? Someplace I read that 5-10-10 was a good fertilizer for them. See, I needed this forum before they were a foot tall!

I have read that dahlia fertilizer should have low or no nitrogen. It causes the plant to make a lot of leaves at the expense of flowers and, as Ernie said, the tubers will rot in storage. I don't know if there is enough Miracle Grow in the mix to do any damage. Maybe an expert will come along to advise.

We're no experts by any means, yet all we use is compost for N, our homemade compost tested out @ 1.5%N, which is basically just enough N to keep compost decomposition going after it's applied to the soil. We do add bone meal and greensand to keep P and K with some trace supplemented. This is only the second year for the 40 or so Dahlias we have. Our soil is deep and sandy so we left them in over the winter and they've all returned, even the bedding dahlias which I assumed were going to behave like annuals. That's what I get for assuming. lol.

We were also told by the Dahlia person we got our tubers from that high N was the eventual kiss of death for the tubers.

It's wonderful that this forum exists so all of us can share these amazing flowers.

Candy the fawn was at Ernies. Try to visit this time before you leave lol maybe we can both be home that way ha ha. Ernie

And maybe it won't be raining!

drdon - LOL The bonemeal would certainly be the kiss of death for my Dahlias. Not because of any chemical problem. My puppy digs up EVERYTHING planted with bonemeal. I just lost a rose bush to bonemeal. :-) But you have the magical ingredients to make everything grow.

One site suggests using the same type of fertilizer they do on potatoes.
It looks like your miracle grow shouldn't be a problem although he does recommend more nitrogen than I would expect.
http://www.rittenhouse.ca/hortmag/Horticulture&Gardens/July01/Potato.htm

I've been feeding my Dahlias the same thing I feed my Iris. 5-10-10. I'd use bonemeal too - but I've had the doggie problem.

They recommend cow manure but my guess is that they didn't have access to horses. :-)

I do think that any bulb or rhizome that is flowering needs a low nitrogen fertilizer as a general rule.

This message was edited Jun 6, 2005 11:14 AM


Comments (13)

Goren

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.


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