The Garden Professors™

Last month we discussed the various heavy metals that might end up in your garden and landscape soils. Today we’ll consider how different factors can alter heavy metal uptake by plants.

First of all, let’s consider plant uptake. Plant roots can either accumulate a particular metal or exclude it. If they exclude it, that’s the end of the story, (though it’s still a soil contaminant). If plants take it up, they can either store it in their roots, or they can transport it to some other part of the plant – stems, leaves, flowers, and fruits are possible destinations for metals in accumulator plants. Accumulation varies with plant species and life stage; in other words, seedlings may have different uptake abilities than later life stages. And of course, whether a plant accumulates or excludes a particular heavy metal does not mean the same uptake pattern holds for other heavy metals.

Secondly, soil conditions will influence heavy metal mobility. Heavy metals are positively charged, so anything in the soil that carries a negative charge – like clay particles and organic matter – will tend to hold heavy metals in place. That can either be good or bad, depending on your use of the landscape. If you are growing edibles, metals that are tightly bound to the soil are less likely to be taken up. But this also means that they are pretty much there to stay. Sandy soils don’t hold metals well, since sand particles carry no charge, so heavy metals are free to move elsewhere – into the air, into bodies of water, or into plant roots.

Additions of fertilizers, like those that contain phosphate or that chelate metals, will also increase the ability of plants to take up heavy metals. Likewise, earthworms ingest metals and bind them to other compounds that can be taken up by plant roots. And microbes associated with the roots (and the roots themselves) can acidify the rhizosphere, solubilizing metals and making them easy to incorporate.

It’s apparent that many factors are at play in determining whether plants will take up heavy metals, thus making it impossible to come up with lists of “safe” plants. There are hundreds, if not thousands, of studies on heavy metal uptake of vegetable and other crop plants worldwide, and the variability among their results is a direct reflection of the complexity of soil environments and plant physiology. Nevertheless, there are some very general observations about accumulator species that can be gleaned from the research:

1. Roots are the most likely tissue to contain heavy metals, since they are the point of uptake; arsenic can accumulate in carrots and lead has been found in carrots and potatoes;
2. Stems are much less likely to accumulate heavy metals, as they are basically just a straw connecting roots to leaves and other terminal tissues;
3. Leaves, including basil, lettuce, and spinach, can accumulate heavy metals. Moreover, it appears that red leafed cultivars may accumulate more than those that are green leafed;
4. Flowers and fruits, including vegetable tissues that produce seeds, are less likely to accumulate heavy metals. For plants that depend on animals to spread their seeds by ingesting the surrounding fruits and then excreting their seeds, it would be an evolutionary disadvantage to have those tissues carrying toxic heavy metals. That being said, there are vegetables, like beans, broccoli, and zucchini, that can accumulate heavy metals such as lead and arsenic.

By this point, I think we can agree there will never be a “one size fits all” approach to gardening safely when heavy metals are part of the soil, water, or air environment. Next month I’ll provide suggestions on how to navigate the confusion and design your own approach to creating gardens and landscapes that work around heavy metal contamination.

Our first major frost hit my part of Arizona a month ago, killing all tomato vines. I did my thanksgiving cleanup chores–removed all the vines and ground them into mulch. I noticed an ominous symptom on one a few of the heirloom varieties (Prudence Purple) that I removed—galled roots. This symptom when seen on tomato is evidence of Root Knot Nematode (RKN). More about RKN shortly. Nematodes are non-segmented worms, mostly free living in soil and feed on bacteria, fungi, small animals or each other. Nematodes are small, barely perceived without magnification but easily observed under low power microscopy. Most nematodes are principal components of the soil food web and are vital to its health and functioning. A few kinds (>30) are opportunistic plant feeders. Plant pathologists consider nematodes plant pathogens because they evoke complicated responses in plant physiology leading to the development of symptoms.

Plant parasitic nematodes have some common features and some rather diverse feeding habits and lifestyles. All plant parasitic nematodes have a stylet or spear at their mouth end that is used to puncture plant tissues and such the sap from their host. Looking under a dissecting microscope you may not be able to identify the genus of a nematode but you can tell if it is bad for plants by seeing the spear just behind its mouth. Plant Parasitic Nematodes (PPN) are either migratory or sedentary. All PPN reproduce by eggs and molt once inside the egg emerging as a second instar juvenile nematode. After a couple more molts the juveniles become adults. Male nematodes are less common than female worms. As adults they can keep feeding from plant to plant if they are ectoparasitic (feeding outside of the root) or they can settle down and make eggs inside a cyst or gall. Some nematodes are endoparasitic and once inside the root never leave it until their eggs hatch and juveniles swim off find another host.

Gardeners should be on the lookout for PPN by noticing symptoms of infection. The most common symptom caused by nematodes is stunting or reduced growth. There may be no other symptoms observable. When the number of PPN is quite large, yellowing or chlorosis can occur as the worms shut down a plant’s ability to take up water and minerals. RKN is the most common destructive plant parasitic nematodes for many gardeners. The gall symptoms on roots are indicative of an infested host. Galling can be light or complete, occurring on every root the plant has. RKN survives in soil for years even without a host because the eggs enter a dormant stage called cryptobiosis. Hatch is snychronous with susceptible roots that grow nearby. Root knot nematodes can build huge populations in a single growing season. Gardeners get nematodes by introducing contaminated soils that come with plants to their gardens. Since symptoms don’t show on plants with minor infections, people think they are buying healthy stock. Even with RKN, there may be juveniles in the soil that have not formed galls yet and when introduced to your garden they will develop later on susceptible plants.

RKN has a very wide host range. Fruit trees, impatiens, calendulas, and tomatoes are a few of its common hosts. Perennial plants can really develop high populations of RKN because the host is undisturbed and provides many seasons for the pathogen to develop. Once detected as galls on roots the plant should be removed and destroyed. RKN is particularly horrible for tomatoes and other annuals when it combines with fungi that also cause disease. RKN forms disease complexes with Fusarium which causes wilts. When tomatoes are infected with both RKN and Fusarium the symptoms are severe, and the plant will die relatively early in its life cycle often before a crop can develop.

Chipping or grinding and composting will kill most nematodes if you want to reuse your greenwaste. More likely RKN will survive as eggs in the soil. Soil samples that find just one RKN per gram of soil sample are considered hazardous as the worms can rapidly develop from these low populations. You may have heard that Marigolds will control RKN. Switching gardens to a non-host (crop rotation) does help decrease populations. And French marigolds and crucifers if tilled into soil as “green manure” will decrease RKN but these methods will not eliminate them from soil. There is a dose response to tilling in mustards so the more you incorporate the more RKN will be harmed. Some varieties are better than others. Fumigation provides a good level of control but is not feasible outside commercial agriculture. Soil solaraization with plastic tarps also controls nematodes in the upper regions of soil but there are usually many eggs that survive in lower soil profiles. The best control is not to plant susceptible plants.

Some tomato varieties are resistant to RKN. In fact VFN (Verticillium Fusarium and Nematode resistant) varieties should be chosen to avoid recurrent problems. The resistance to RKN in tomato is not complete and under high nematode populations and/or high temperatures the resistance can break down and even resistant varieties can develop galls and symptoms. There are no pesticides that home gardeners can use to kill nematodes. However there are biological controls of nematodes and since they are soil food web opportunists, increasing the diversity of organisms in soil tends to cut down on PPN. As always, fresh arborist chips applied as mulch will build a resilient soil food web and will slow the development of PPN harmful to garden plants.

Summer is done. The last apples are coming off my orchard trees now and persimmons are ripening fast. Some fruit remains to be picked but most is off. As garden productivity subsides we turn our tasks to winter. In Southern California it means planting the winter vegetable garden, in Northern Mn snow has already flown so gardens are shut down now. For fungi that may be pathogens in our gardens, it is a time for reproduction. Fall is the time for fruiting and for gardeners a time to reckon with next year’s disease cycles.

Most fungi are saprophytic, that is they live on dead or decayed organic matter. Fungi are largely responsible for recycling forest nutrients from litterfall (leaves, branches and whole trees) back to soil minerals. Without fungal decay, mulch would never break down and organic matter would pile up. If you use fresh wood chips (often advocated in this group) you may notice that after some time they are full of fungal mycelium or cordons (rhizomorphs). This is normal and healthy—a good sign that your mulch is decomposing and improving the underlying layers of soil.

Some plant pathogen fruiting bodies are edible. The mushrooms formed by Armillaria are often collected and considered delectable by many. Most edible fungi are saprophytes or mycorrhizal fungi. Truffles and other edible mushrooms like Chanterelles are plant symbionts often benefiting oaks and other northern temperate trees. Some wood decay fungi are also considered a delicacy such as the Oyster mushrooms (Pleurotis spp.) or the sulfur mushroom (Laetiporus gilbersonii). I don’t recommend harvesting wild mushrooms for food unless you are able to accurately identify what you collect, even then, second opinions of mycologists are a good idea. Also, not everyone reacts the same to fungi when they consume non-commercial mushrooms, so moderation is best or just get your fungus from commercial sources.

Not all fungi are beneficial. Some have evolved life histories that allow them to gain energy not from organic matter or dead plant materials but from living plants. These are parasites. Fungi have been evolving their lifestyles for about 400 million years and in that time have developed several strategies involving plant hosts to live and reproduce. Sixty five million years ago, after the Cretaceous-Paleogene extinction event that famously destroyed dinosaurs, fungi bloomed on earth and increased in importance. As land plants diversified, so did fungi developing many forms and parternships, many of them becoming essential to plants such as mycorrhizae. A few fungi specialized as plant pathogens.

Fungi use their reproductive structures to survive and ready themselves to attack susceptible plants. The most common fungal fruiting body the mushroom may not seem like a survival structure. But mushrooms can produce millions if not over a billion spores. Massive spore production ensures that some of those spores will find a place for the organism to survive. Also some mushrooms found on trees (sometimes called conks or bracket mushrooms) are perennial, and live for years—each year they add a new spore bearing surface over the last one. Many of the pathogenic tree fungi that produce conks fruit in the fall or winter.

Many fungi form their fruiting bodies as small melanized structures that contain their spores. These are often formed in dead host tissue, such as dead twigs or branches. The spores are protected until they are splashed by water onto tender or susceptible plant tissues such as shoots. In soil, fungi can form hyphae that are very concentrated and melanized in to long lasting structures called scleortia. They lay dormant in soil for years until a susceptible root grows into them. Crop rotation often helps to limit disease but some fungi can last decades between crops and remain viable by producing thick walled spores called chlamydospores or sclerotia. The wilt fungi (Fusarium and Verticillium) survive in this way.

Another key strategy that fungi use is a kind of timing called phenotypic synchronicity. Fungi often have their spores ready to be dispersed exactly when new growth or susceptible plants are available for infection. The timing also often aligns with weather conditions that favor spore dispersal or arrival at the intended plant growth stage or phenotype.

Fungi evolved with land plants to take advantage of the environmental conditions and phenology of their hosts. We can interrupt the process with a bit of diligence as gardeners. As fall continues and winter approaches, it is a good time to remove dead twigs and branches from perennials that are prone to disease, clean up fallen or dead flowers from plants like Camellia that are attacked by petal blight because the flower mummies contain sclerotia that start the disease in the Winter. Unfortunately removing conks from trees does nothing to stem the progress of wood decay fungi in the tree they formed on, or their further spread, because so many spores are formed that the few mushrooms we remove will not stop those diseases. Some evidence suggests that increasing soil organic matter will over time reduce soil-borne pathogens, but once a pathogen has affected a perennial, there is often little to be done about it as in the case of Verticillium wilt of shade trees. No matter how fungi survive, its always a good idea to apply fresh tree trimming chips around perennials in the garden….

Extremes

Here in California we had an extreme heat event on September 6, 2020. In my yard temperatures peaked at 120 degrees F. This also happened back in 2018 earlier in the summer where we reached a similar peak temperature. It is not supposed to get to be 120 degrees F. in Ojai. This year new high temperature records were set all over southern California for the month of September. Following these heat extremes, wildfires have spread from border to border (Canada to Mexico) in western states. As we suffer through heat and flames here in Western US states, we are also now told that this is a la Nina year so Southern California will continue with drought conditions into 2021. Extremes in climate bring hot dry weather to the Western United States and hurricanes and drenching rains to the eastern United States. Plants in landscapes may or may not be adapted to these extremes.

My poster child heat monitor is the coast live oak, Quercus agrifolia. When temperatures exceed triple digits >110F, foliage on this native oak turn brown and burn on the south exposed canopies. They are not adapted to these record temperatures. This can be evidenced by looking at the damage throughout many California communities. Coincidentally other non-native plants are better adapted to high temperatures. The California pepper or Peruvian Pepper (Schinus mole) does fine in 120F weather with no irrigation. Eucalyptus of several species also have tolerated these increased temperatures. Trees that are drought stressed from lack of irrigation after a long dry summer will sunburn more severely than the same plants under consistent irrigation. If you see this kind of damage, its best to leave it alone until the plant responds by growing new shoots.

While study of “climate ready” trees is giving us tree selection options for hotter climates, the research is still new and we have many other species to consider beyond what has been recently reported. Of the species I have in Ventura County few of our study trees showed any damage from the extreme heat, and only the very youngest leaves were damaged on western hackberry and Catalina Cherry. Pistache, Island Oak, Palo Blanco, Tecate cypress, Arizona madrone, and Ghost Gum were not affected by triple digit weather this September. Other ornamental species that were damaged all over Southern California include the following: Avocado, Camphor, Privet, Magnolia, Coast Live Oak, Sycamore (especially the native Platanus racemosa), loquat and ornamental plum.

It our recent heat damage surveys I have observed that Coast Live Oak and Western Sycamore, two native trees that enjoy widespread tree ordinance protections were consistently damaged by our hot day early this month. If we continue to have extreme hot days, poorly adapted native trees will be injured more frequently, and possibly become more susceptible to damaging insects or native pathogens. This tends to restrict the range of natives to areas they are still adapted to growing in or grow into a new region where they are more successful. A time may come when a native tree is not the best choice for your area.

McPherson E.G., Berry, A.M., van Doorn, N.S., Downer, J, Hartin, J., Haver, D., and E. Teach. 2020. Climate-Ready Tree Study: Update for Southern California Communities. Western Arborist 45:12-18.