Question
Hi - Thank you so much for solving my problem with non flowering geraniums and now for non flowering dahlias. My dahlias flowered beautifully at first but gradually the flowers died and the plants no longer flower. They are in the same pot as the geraniums. The fertilizer I was using was high in nitrogen. Could that be the problem with the dahlias as well?

Answer
In the world of Kingdom Plantae, much depends on the weather.  In the Summer of 2010, over most of the Northern Hemisphere, growing conditions are defined by boiling heat waves.  Over much of the planet, flowering problems are not the exception.  They're the rule.

Generally speaking, Nitrogen does drive excessive growth of foliage.  Over-fertilize a Petunia with Miracle-Gro and you'll be looking at a lush, green groundcover.

Some plants, like Roses, need enormous amounts of energy to build their flowers; they need A LOT of leaves. A LOT of light.  A LOT of leaf-building Nitrogen.

Geraniums and Dahlias are a little different.  They need less 'food' than very heavy feeders like Roses for their construction projects.  Their petals and flower parts, their leaves, their fragrance (nil), their pigments and, ultimately, their seeds are simpler to build.

But they do need nutrients.  And they are especially needy when it comes to Nitrogen, the most common element in the atmosphere.  Unlike true Annuals (Petunias, Sweet Alyssum, Zinnias), Dahlias and Geraniums are tender Perennials that require Nitrogen.  Too much, and you end up with a lovely specimen of foliage, and no flowers.

But this is the Summer of 2010.  There is more to this.

Under common growing conditions, Geraniums and Dahlias are both a piece of cake.  Temperatures this Summer have proved to be extreme environmental variables that people rarely deal with.

Here's the problem.

Temperatures at day and at night are very important to plants because they control hormones that decide exactly how these plants are going to spend their day.  Extreme temperatures turn them on and shut them off -- protective mechanisms to raise their odds of survival, which is what happens in dormancy.  That includes, in the case of both Geraniums AND Dahlias, heat waves in the mid-80's F and up. Any time you sweat, assume your Geraniums and Dahlis are in trouble.

One problem these poor plants have with high temps is that the heat damages Thylakoids in their cells.  Thylakoids play a direct role in photosynthesis.  How well any plant can take heat stress without damage to its Chlorophyll actually has a name: It's called the "Stay Green Trait" (identified and named in 1997).  Note that the plants that earn that distinction are considered 'special' for their ability to grow when temps are in the 80s for extended periods; temperatures in the 90s are too tough for THEM.

Remember that the Stay Green Trait label identifies these particular plants as 'experts in thier field'

Not only are your Summer flowers ill with Chlorophyll damage, they're also not able to bloom.  Because heat does something else.  It controls flowering.

Geraniums, as you already know, set buds anywhere from 45 degrees F and up -- until you hit 80 degrees F.  Ideally, optimum growth, habit and flower formation is found when it's 70 to 72 degrees F day and night.

Dahlias are a little different.  Short days are what induce flowering in these plants.  As Nexus Greenhouse Systems warns in their report, 'Manipulating Dahlias', 'Critical day length for tuberous root formation is 11-12 hours.'  Growers have to know this to get them to bloom, because no one will buy them if they're not blooming:

www.gpnmag.com/Manipulating-Dahlias-article2850

Dahlias need a little more Nitrogen than Geraniums, so although too much Nitrogen is not a good thing for any plant's flowering habits, there are other, potentially stronger forces that get in the way of a mid-summer bloom boom.  Look at this complicated problem a Dahlia grower has to deal with to stay in business:

'Short days induce flowering in Dahlia...Longer day lengths are required for proper flower development.

'Konishi and Inaba (1964 and 1966) found the optimum day length for flower induction to be 10 hours or less; however, they determined that a day length of 12 hours ?or greater was required for proper flower development. They observed that plants grown under continuous, 10-hour photoperiods had a high percentage of aborted flower buds.

'Halburton and Payne (1978) showed that while long days delayed bud set and flowering, the flowering percentage and overall flower and foliage quality of Dahlia 慠edskin?actually improved with long days.

'Plants grown under short day lengths were shorter than those grown under longer day lengths, suggesting that photoperiod could be used as a method of height control.

'Durso and De Hertogh (1977) observed that natural springtime photoperiods increasing from 10-14 hours were optimal for forcing of the tuberous rooted cultivars 慘olchelsee?and 慞ark Princess? Brondum and Heins (1993) determined that the optimum photoperiodic conditions for the production of 慠oyal Dahlietta Yellow?were 12-14 hours at approximately 20?C,' or 68?F.

Remember, the Dahlias you are concerned about (and I assume were purchased as blooming seedlings) were un-naturally programmed to develop flowers.  Without those short day treatments to push the ON button for the flower cycle, those plants have simply been using up what they'd already made.  Now they don't know what to do.

Here's my prediction.

As the days continue to grow shorter, but more important, as they grow COOLER, you will find more buds and, soon after that, more flowers on BOTH your Dahlias and your Geraniums.

Don't overfertilize.  Using organic fertilizers will be kinder and gentler, but a high quality potting soil (the kind you make at home) will do wonders for them.  Stick an Earthworm in each pot if you can.  Keep them in the Sun, pinch off spent blooms, and be patient.  Good things come to those who wait.

Remember this experience next Summer.  Once temps hit the 80s, with no relief, expect Nature to take its course.  Heat can be brutal.  If scientists are correct, we are in for a hell of a ride.