Timing Is Everything

Bumblebee nectaring on native Garberia heterophylla

Hummingbird clearwing moth nectaring on a nonnative Buddleia

As I've written before, my Holy Grail of pollinators in my new landscape is the continual presence of bumblebees. For the past week, I've had one in my yard and it shows up each early morning to visit my patch of native red salvia (Salvia coccinea). It is not the plant I would have guessed that would entice the visits, but I am excited to see it. I don't see the bumblebees later in the day yet, but they visit me routinely in the morning hours - and only on this plant although I have dozens of blooming wildflowers for it to choose from.
I've learned over the years that timing is everything - much more important than the exact species of flowering plant. Plants are not identical in what they provide and they do it much more "intelligently" than most folks seem to understand. As I have more time right now to watch the pollination of my flowers, that simple fact is more apparent than ever.
Plants entice pollinators with their nectar and pollen, but they do not do it haphazardly. I write about this much more thoroughly in my new book, The Nature of Plants: An Introduction to How Plants Work. Nectar and pollen are valuable resources and cost plants a great deal to produce. They are not products a plant can afford to waste. Their entire future rests in having it work effectively. If it doesn't, they leave no progeny behind.  They time their production carefully and they dole it out over the course of the day in a way to best-assure pollination. Evolution works that way. The "smartest" succeed. The rest decline and eventually disappear.
Nectar is a cheap reward for all that pollinate a flower. The sugar solution provides a high energy reward, but it is short-lasting in its benefits. This is not an accidental fact of life. Pollinators are "forced" to return over and over again to maintain their basic metabolic needs. Producing nectar, however, is not cheap for the plant. It takes away the sugars produced during photosynthesis that would be used for growth, producing hormones, and fighting off pathogens.  Because of that, most plants produce nectar only when it is going to be most effective for pollination. Some time their nectar production to times of day when their pollinators are most likely to be active and they turn it off when those pollinators are least likely to be visiting.  Few plants produce nectar after dark. Those that do, are moth pollinated and don't produce it during the day. If you watch your flowers as often as I do, you may notice that visitation rates are not the same throughout the day. Different potential pollinators are not often active at the same times and plants may well time the release of their nectar to entice some of these pollinators more than others. All plants, regardless of this, release nectar only until their floral tubes are full. Once they have done this, they wait until it is drained. Plants have to be smart about this. Nectar does't ever seep out over the floral tube and rain uselessly down the stem and out onto the ground. If the nectar is sipped by a pollinator, the flower will produce more, but not until then and never more than what is needed.
If a flower is pollinated (a very different phenomenon than simply being visited), the flower will stop its production. There is no sense in wasting nectar once its purpose has been achieved. Pollinators may revisit a flower many times over the course of a day, but once pollen is deposited on its stigma and the pollen tube starts burrowing down the style towards the ovary, the flower stops nectar production.  It is an economy vital to the reproduction process.
Pollen itself is even more costly to produce, but it is the entity required for reproduction.  Bees purposely take it for its food value and feed on it or bring it to their queens or offspring. Flowers release pollen knowing that some of it will be consumed, but that some will get transferred to another flower and ensure that their progeny will get 2 different sets of chromosomes. Animals rarely reproduce willingly with their close relatives. Plants have a number of strategies to do likewise. Most release pollen at different times than their stigmas are receptive to receive some.  The transfer of pollen often only lasts a brief time while the production of nectar may last days longer.
A well-designed pollinator garden provides a wide diversity of flowers to ensure that a wider diversity of pollinators get what they need. Not every flower attracts the same attention as every other. A well-designed pollinator garden also includes flowers for as long a time period as possible. In that way, different plants are not all competing for the attention of the active pollinators present at any one time. There are spring-blooming to late-fall-blooming plants for a reason, just as there are plants that are most-actively pollinated in the morning and those that wait until later in the day to be their most enticing. 
Timing is everything. Watch your plants and their pollinators throughout the course of a day and I suspect that you'll see the subtle changes that occur within your landscape and you'll notice that different pollinator species are most-active at different times - or that they will visit different flowers at different times of day. Natural selection weeds out the most unfit and it creates an almost-magical balance to behold. Once you learn to see it, it makes a pollinator garden a much more fascinating thing to behold.