Friday, August 19, 2011

The Dog Days of Summer

Lush green prairie in Devil's Backbone open space, near Loveland, CO on June 15th.

Same view on August 19th!

It is hard to believe, but school is about to start again in many parts of the country, and soon it will be fall with cool nights and wonderful colors. But it is just August 19th today and it still feels like the middle of summer. The hot days of summer are often called the dog days going back to traditions that started in ancient times. It was once believed that these hot days in August were due to one of the brightest and closest stars shining down on the Earth at the same time as the sun. This is the Star Sirius which is the brightest star in the constellation Canis Major or "big dog". Hence the dog days of summer. But actually the heat of summer just like the bitter cold of January and February is simply a consequence of the ability of the atmosphere to hold and retain heat (or cold), in this case over the course of a hot summer.

Anyway in these hot days of summer there are still plants out there that begin to flower, and some are key budburst species. It may seem odd for plants to wait until nearly the end of summer to flower, but there are often advantages to doing it this way. In the Rocky Mountains and Great Plains one common group of plants that flowers very late are the sagebrushes, including the big sagebrush (Artemisia tridentata), wild tarragon (Artemisia dracunculus) and prairie sagewort (Artemisia frigida). In Montana they first flower in early September. I would expect them to flower a little earlier further south in the Rockies in Colorado, but so far this year this does not seem likely. So what is the advantage of this very late flowering? Given that all these plants typically grow in dry places with long cold winters, and moist early springs, it may be that late flowering works well since they can quickly develop fruits that disperse by wind well in time for the fall storms, then the seeds will be ready for optimal germination in the spring. They are all wind pollinated, so the abundance of pollinators is not an issue either. If they are dispersing fruits after the leaves have fallen from deciduous shrubs or wildflowers, then the wind will be more effective at dispersing these fruits as well.

The other group that seems to often flower this time of year are agricultural and garden weeds, many in the sunflower family. It may be hard to find virtue in these plants that are the bane of every agriculturist. But they are certainly well adapted to what they do, and there would appear to be distinct advantages for them to flower late as well. It is well known that these species have literally evolved very quickly with our technology and approach to managing gardens and fields. This is easy to visualize when you think about any plant which somehow misses our trowels or hoes or chemicals is much more likely to grow and reproduce than those that succumb to our efforts to control them. In August many gardeners tire of the task of keeping up the rapidly growing garden and its prodigious harvest, resulting in letting many weeds go. Late flowering and fruiting may be a big advantage for these weeds!

One budburst species which is a common lawn weed flowers much earlier than other plants and can become well established before lawn grasses are active, providing an effective way to persist mowing later on. White clover (Trifolium repens) has long roots where many nutrients are stored, so that even if you pull the plant and get many of its roots, the remaining roots will have plenty of resources to quickly grow new leaves and stems. This same strategy is the hallmark of a spring and summer lawn weed, field bindweed (or common morning glory). I have spent many days pulling almost all of these plants out of the garden, only to find their survivors growing back at breakneck speed.

In moving to Colorado I have been introduced to quite an amazing weed called the puncture vine, or goathead (Tribulus terrestris or "terror of the Earth"). Amazing was not actually the first word that came to mind here after it punctured all my bike, garden cart and wheelbarrow tires. It turns out that this plant has a very unique fruit and structure so that it actually has its own family, the caltrop family. So what is a caltrop? This is an example of what I would call a form of "biomimickry". It turns out that this fruit has a unique structure. It has sharp thorns that are arranged such that no matter how it is blown or tossed about it will always have at least one sharp thorn facing upward, and braced by the others on the ground. This same structure has been used to make weapons designed to puncture animal hoofs or tires of vehicles since Roman times. Just this week I noticed its fruits are ripening, and will be ready for action soon.

Notice the brown fruits that are ripening on this puncture vine. It often spreads out onto trails for maximum effect!

If you live in a dry place you may already be seeing fall colors or at least leaves turning yellow or brown. Just today in a nearby shortgrass steppe or foothill scrubland I noticed that mountain mahogany and wax currant are losing their leaves. Wax currant actually has already achieved the budburst definition of leaf color (50%). Bitterbrush (Purshia tridentata), a budburst focal species should also be dormant by now, and will actually put out new leaves not in the spring, but in the fall. Again an adaptation for the pattern of weather in intermountain regions.
This wax currant (Ribes cereus) already has less than half green leaves

Nearly every month of the year in many parts of the country there is something to observe by looking at plants outside, that will give you some appreciation of the many amazing ways in which plants can adapt to the challenges that nature provides. So check out what is going on in your neighborhood, and you may find some interesting new things, and perhaps gain more respect for the many adaptations that they have. There is plenty to observe before fall colors come our way!

Paul Alaback