The Heat is On!

We are in the dog days of summer! Record high temperatures across the country have been reported. Let’s face it, climate change is real and the planet is getting warmer, despite record cold temperatures in winter in some areas. With climate change, there are more occurrences of extreme temperatures as well as precipitation such as severe drought, flooding, heavier than usual snowfall, etc. So, how does climate change affect gardeners? To understand the concept, we must first understand how high temperatures affect plant growth and development.

Plant temperature tolerance is the ability of any plant to adapt to a given climate at both low and high temperatures. It is obviously important to use species that not only can survive our winter temperatures, but also our hot summers. Similar to the U.S.D.A. Plant Cold Hardiness Zone map (, the American Horticultural Society published a plant heat zone map ( The heat zones are based on the average annual days with temperatures above 86°F (30°C). For example, up in my neck of the woods in Wisconsin, we have four heat zones in the state; zones 2-5 with the cooler (lower number) heat zones in the northern part of the state and the warmer heat zones in the southern and western part of the state and in urban areas. In comparison, North Carolina has seven heat zones (2-8). California has even more heat zones due to its varying climates and elevations.

Unique microclimates can be created at a specific site. Urban areas usually are slightly warmer than rural areas due to the heat island effect created by light reflection off of buildings, heat rising from sewers, and trapping of heat between large buildings. For example, trees planted close to pavement have warmer conditions in summer, than trees planted near turf, especially in the root zone, and have greater water needs. Trees and shrubs in this environment should receive, but rarely get supplemental irrigation compared to those same plants planted in the middle of a lawn or park area with cooler root areas and less heat stress to the tree.

Street trees compete with other plants and are subject to heat and drought stress
Street trees compete with other plants and are subject to heat and drought stress

So what does this mean for temperate woody plants?

High temperature stress is important for a number of reasons. The main effect is increased water use. However, water availability is often limited, especially in urban environments, leading to a net loss of moisture within the plant. Leaves loose water quicker through the stomates via transpiration as temperatures rise. The stomates then begin to close and the cooling effect of evapotranspiration is stopped.

Other physiological processes are impacted by high temperatures such as fewer carbohydrates available for plant growth and development, generation of plant pigments (red, purple and blue pigments) and defense used in protecting plants against insects and diseases. When plants with poor heat tolerance are grown in regions that routinely experience high summer day and night temperatures and insufficient moisture, plants will use many of their stored sugars during the evening hours and during the hottest part of the day. For many temperate woody plants, optimum temperatures for photosynthesis are below 86°F (30°C). Above this temperature, net photosynthesis declines with increasing temperatures. If this continues long term, plants can die, especially young plants that do not have many stored carbohydrate reserves and are under drought stress.

'Ames' Kalm's St. Johnswort (Hypericum kalmianum 'Ames') is heat and drought tolerant
‘Ames’ Kalm’s St. Johnswort (Hypericum kalmianum ‘Ames’) is heat and drought tolerant
Black chokeberry (Aronia melanocarpa) is a three season interest, heat and drought tolerant plant
Black chokeberry (Aronia melanocarpa) is a three season interest, heat and drought tolerant plant
Purpleleaved ninebarks (Physocarpus opulifolius) are very drought and heat tolerant
Purpleleaved ninebarks (Physocarpus opulifolius) are very drought and heat tolerant


What can we do as gardeners?

Unfortunately, climate change is occurring at a fast rate and each decade is getting warmer and often drier than the preceding decade. Here’s what we can do as gardeners:

  • Select non-invasive, pest resistant, stress tolerant plants for your landscape
  • Non-drought or heat tolerant landscapes plants will require regular watering to sustain them
  • Use of soaker hoses or drip irrigation is better than overhead watering as the moisture is directly applied to the roots with less runoff
  • A plant is not truly drought tolerant until it has been established in the landscape for at least three years or more to allow for root growth, especially for trees
  • Some native and exotic plants can tolerate these changes, but some native species in natural areas may be lost if our climate continues to get warmer
  • Incorporate compost into the soil prior to planting new areas to improve moisture retention and aeration
  • Apply 2-4” of bark or wood chips to the top of the soil to retain soil moisture. Make sure the mulch does not touch the trunk of trees or base of shrubs. Excessive mulching (mulch volcanoes) can actually kill landscape plants by impeding air and moisture penetration and invite fungal diseases.
  • Cities need to avoid planting trees in tree pits (sidewalk cutouts), i.e. restrained planting areas in sidewalks along streets. These trees are under severe moisture, drainage and heat stress and do not live long. Planting areas that are wide and long work much better for tree health and longevity than planting in tree pits.

Laura Jull, Ph.D.

University of Florida study: Mulch reduces soil water loss to evaporation by 33%

I was just in Fargo for the North Dakota Urban and Community Forestry Association conference – our pantywaist -12 deg. F wasn’t cold enough – I wanted to experience some real winter. Dr. John Ball from South Dakota State University, whom I enjoy listening to, was also on the program. As an aside, if you are ever invited to speak at the same conference as John, DO NOT allow yourself to be scheduled after him. He is hilarious and you will sound like a boring moron by comparison. John is an excellent and entertaining speaker and I usually agree with 90+ % of what he says. In this case he was talking about mulch – a subject near and dear to my heart – and lead off by mentioning the recent study by Gilman et al. (2012) as a reason why we should be concerned about possible negative impacts of mulch.

Oh boy. Let’s go through this and see what the paper does and doesn’t say about mulch.

The study was done in collaboration with Richard Beeson, who is widely known for his work using weighing lysimeters to estimate water use by container-grown trees and shrubs. In this case lysimeters were used to measure water lost from containers filled with soil or container media (60% pine bark : 30 % peat: 10% sand) and then covered with pine bark mulch or left uncovered. The containers were watered and allowed to drain. Water loss due to evaporation (there were no trees in the containers) was measured for three days. For the first day after watering there was more evaporation from the containers filled with container substrate that were mulched than the containers that were not mulched. However, if you add up the evaporation over the entire 3 days there is no difference between mulched and non-mulched containers. This is not too surprising since the mulch and container media were largely the same thing – pine bark.

Now here’s the important part. If you look at evaporation for the soil-filled containers – which are the ones we really care about from a landscape perspective – total evaporation was 3L for non-mulched containers versus 2L for mulched. In other words, there was much greater water loss from soil when the pots where not mulched. A much better title for the article would have been, “Mulch reduces evaporation from soil by 33%”!

gilman pic

It’s easy to nit-pick articles but this isn’t pit-picking. There is only one table in this article and they got it wrong. For whatever reason people are prone to hysteria when it comes to tree care and I can already see this morphing into “Oh my God! Did you hear? Ed Gilman says if you mulch a tree you’re going to kill it!” A misinterpreted 3-day study has everyone criticizing mulch while ignoring a vast body of long-term studies.

For Mulch

Posted by Bert Cregg

Just a quick note up front that today’s post is a little data heavy, so if you’re still adjusting to this weekend’s time change; be advised.

A few weeks back Jim Urban wrote a post entitled ‘Against mulch’ on the Deep Root blog. The principle reasons he cited for his position were: 1) Mulch floats and can clog drains and releases “lots of phosphorus” as it breaks down, and 2) work by Gilman et al. that suggest that mulch does not reduce evapo-transpiration. We discussed the Gilman et al. paper ad nauseum here already so I’ll stick to the other points.

Most organic mulches float, it’s true. However, if mulch is repeatedly washing from a bed into a drain this suggests a problem with the design as much as anything. Second, I’m not sure what constitutes “lots of phosphorus”. Branch and stem tissue of hardwood trees is about 0.1% P. If we use just the bark as mulch, the P concentration is about 0.2 to 0.3%. Is that ‘lots of phosphorus”? I don’t know. I suppose if you put enough it down and allow it wash into a drain it could be.

So let’s stick to what we do know about landscape mulch. Linda has written the most comprehensive review of mulch out there and it demonstrates the benefits of mulch. Nevertheless I’d like to add some recent observations of my own to the discussion. These come from follow-up measurement on some studies that we have already published on shrubs and conifers. But I think our new data are important because they demonstrate the long-term benefits of much on tree and shrub growth.

2006 Conifer study. In 2006 we installed a trial to compare several different weed control strategies for newly planted conifers. Weed control, either by hand, Vis-pore mulch mats or 3” of coarse wood chips, dramatically increased tree survival.
swmrec mulch survival

After 8 growing seasons, trees that had the wood chip mulch or mulch mats had significantly greater caliper than trees that were not mulched.
swmrec mulch caliper

2004 shrub study. In another trial we compared the effect of various mulch types (wood chips, pine bark, hardwood bark) on growth of common landscape shrubs (golden globe arborvitae, Runyan yew, ‘Tardiva’ hydrangea, cranberrybush viburnum, and arrowwood viburnum). We re-measured heights of the shrubs study a couple of weeks ago (nine growing seasons after installation). To keep things simple here I’ve lumped the mulches together and simply compared mulched vs. un-mulched.

After nine years mulching increased height growth for all shrubs except the arborvitae.
mulch 2013 ht

Even more interesting is that the growth benefit of mulch extends beyond the establishment phase. If we start at age 4 and look at the relative growth rate for the past five years (i.e., growth increment for past 5 years / height at 4 years) we see that mulch continues to provide a growth advantage for all shrubs except the arbs.
mulch RGR

As I said at the outset, a little data heavy today but I think this is an import point. There is a lot of discussion these days about proper planting techniques but I think after-planting care often gets overlooked and mulching is an important part of that. That’s why I’m for much.

Toxic mulch: When shredded bark goes bad

We typically think of mulching landscape beds as a good thing.  And it usually is; helping to conserve soil moisture, reducing soil temperatures and contributing to soil organic matter.  Recently, however, I received an e-mail from a local landscaper that reported severe damage to annuals and perennials in a landscape bed immediately after applying hardwood mulch.  The problem, sometimes referred to as ‘sour mulch’ or ‘toxic mulch’, occurs when mulch is left is large piles and undergoes anaerobic conditions.  This results in the production of acids and other compounds that can volatilize when the mulch is placed in beds, especially during hot weather.  These vapors can quickly damage annuals and other sensitive plants.  Mulch in this condition is often characterized by a ‘sour’ smell.  If you suspect your mulch has gone sour, spread it out before use to allow toxins to dissipate and water thoroughly either before or immediately after application.  The University of Arkansas Extension has a nice fact sheet in the subject “Plant injury from ‘sour’ wood mulch.

Fried Gerber daisy

Sedums are usually pretty tough…

And, yes, I did steal the title of this post from one of my all-time favorite ‘Far Sides’…


What to do when it’s still raining?

It’s almost May…and it’s still raining. Even for our normally wet spring climate, this has been an unusually soggy year. I’m also blaming the weather on my 3rd or 4th cold so far this year, which has knocked me flat for the last 6 days (which was why I had no Friday puzzle posted). So in between blowing my nose, hacking my lungs out, and generally feeling sorry for myself, I started looking over 10 years’ worth of photos of our home landscape.

You’ve seen bits and pieces of this before in some of my postings. But one of the spots I’m most proud of is the tiny east-facing side yard that originally contained lawn, a lilac, and a border of arborvitae. Within the first few years the lawn came out and plants started going in. In 2004 I’d installed some small rhododendron, a redbud (left foreground), and a whole lot of woodchips:

Since then we removed the lilac (it had been planted too close to the garage and was a powdery mildew magnet), put in an arbor and wisteria (on the right), and added a few more plants (ferns, bleeding hearts, various bulbs and tubers, etc.).  Here it is two (2006) and five (2009) years later:

This year we’ll finish off the area with some flagstone pavers.

One of the main reasons I’m so pleased with this area is that it was inexpensive to redo and it established quickly. We bought the redbud, the wisteria, and the bulbs, but the rest were donations from friends’ gardens, or volunteers that popped up elsewhere in the yard, or plants that someone else wanted removed (like the larger rhody in the far left corner and the dogwood in the right foreground, 2006 photo). The chips were free; the flagstones were a major score from craigslist (free to whomever would pry them up and lug them out).  All the purchased trees and shrubs were barerooted; and root-pruned if needed before planting. Upkeep is minimal except for a bit of pruning and spot watering during the hottest summer months; we’ve lost no plants other than the occasional bulb poaching by squirrels.

It’s just a little bitty sideyard…but I enjoy walking through it every time I’m outside, even in the rain.