Roots are often overlooked by horticulturists but deserve to get more attention. Of course, they are usually underground and out of sight so it's somewhat understandable why they can be ignored. But, roots play a critical role in the life of a plant. They anchor the plant to support the shoots above. They absorb water and mineral nutrients and conduct them upwards. They store carbohydrates and other nutrients that are a source of energy for biennials and perennials as they awaken and grow in spring. A root's tip is where most of the action takes place.
The root tip has overlapping zones: where cells divide, elongate, or form different specialized cells. At the very tip, the root cap protects the rapidly dividing cells known as the meristematic region or meristem (zone of cell division). Behind the meristem, cells elongate and push the meristem and root cap forward into the soil so the root can explore and mine new soil (zone of elongation). And further back, only a fraction of an inch, is the portion where elongation stops and cells become more specialized and functional (zone of differentiation).
Root hairs form in the zone of differentiation and this is where they begin to poke out into the soil to absorb water and mineral nutrients. Root hairs greatly increase the root surface area and therefore increase the ability of a plant to absorb water and nutrients. Vascular tissue (vascular cylinder) is the the piping that helps conduct water and nutrients upward to the shoots. The epidermis forms the protective skin of the roots.
Root hairs are long, thin, single cell extensions from the epidermis. They profoundly increase the overall root surface area and connection with the soil and are responsible for absorbing water and mineral nutrients. Usually they are short-lived, only functional for several days or weeks. So as the root tip advances into virgin soil, new root hairs must be formed continuously. It is important to keep root hairs healthy. The overall vigor of a plant can often be judged by looking at the condition of the root hairs. A nursery scout should remove the pot if possible and look for healthy, usually white, root tips and hairs.
Interveinal chlorosis is indicative of a deficiency in iron in the leaves (and sometimes manganese or zinc deficiencies). But this does not mean that the soil necessarily has low levels of these nutrients. Unhealthy root hairs or the conditions that they are growing might be to blame. Sometimes high soil pH makes iron less available for uptake. Iron is tightly held by soil and must be mined by actively growing roots. When soil is cold in the spring and roots inactive, sometimes iron might not be sufficiently mined and absorbed. Sometimes root diseases such as Pythium and Rhizoctonia might kill root hairs or reduce their functionality.
The last post demonstrated the remarkable ability and unique features of aphids that allow them to rapidly boost their numbers and colonize their hosts in favorable conditions. What about weeds? What features give them the ability to rapidly colonize a potted crop or planted field? Many plants become weeds because they have the powerful trick of producing many many seeds. Seeds are dispersed by wind, insects, animals, or by their association with our nursery tools and machinery. Often, these seeds are long-lived in the soil. Consider these statistics:
Probably some unfortunate graduate students or field assistants in 1954 were given the task to count weed seed of hundreds of common weed species from about 50 plant families in North Dakota. The table above is just a sampling (Stevens 1957). In most cases a single plant, judged to be of average size and growing where competition was low, was harvested at maturity or when a maximum number of seeds could be obtained. The plants were air dried for two weeks or more, threshed and cleaned to re-move immature seeds, empty florets, etc. All of the sampling methods are described in the Stevens reference given below.
So basically left on their own, weeds have a profound ability to produce seed. Some seed are not viable, some germinate immediately, and some persist, perhaps for years, in the soil as a “seed bank”. This bank represents the holdings of weed seeds in the soil. Place a “deposit” of seed in this bank, and your “interest” is compounded in a big way. An interesting experiment with velvetleaf (Abutilon theophrasti) an important weed in soybean crops demonstrated this (Hartzler, 1996).
Velvetleaf is a prolific seed producer and seeds are long-lived. In 1990, replicated experimental plots were planted with soybean and then with one of three velvetleaf densities: 0, 0.2, and 0.4 plants per square meter. In subsequent years, the experiments were maintained in a corn-soybean rotation. Weed densities were determined at crop harvest for four years. As seen above-- even with competition from the crop plants-- velvetleaf density increased dramatically for years following the very sparse initial planting of the weed. There were even some velvetleaf plants seen in the untreated “0” plots, even though the plots were hand weeded to reduce seed production for 5 years prior to initiating the study.
The number of weed seeds in in the soil can range from near 0 to over 1,000,000 per square yard, and most weed seeds are between 0 and 5 years old. A small number of seed can remain viable for decades or more. With this knowledge, one of the most important principles of weed management is to “never let weeds go to seed”. Never.
Stevens O.A., 1957. Weeds, Vol. 5, No. 1 (Jan.), pp. 46-55
Hartzler R.G. 1996. Velvetleaf (Abutilon theophrasti) Population Dynamics following a Single Year's Seed Rain
We are all familiar with aphids, especially in the spring when their populations seem to increase so rapidly and may require control before plant damage occurs. Aphids have unique strategies to flourish. The aphid mother you see this spring is actually a grandmother. Her daughters and granddaughters are contained within the same body! She does not need sex to produce offspring. The individuals develop within her body and are deposited as live young. In this way, one female may produce as many as 80 individuals per week. It's a strategy that builds up aphid populations quickly to take advantage of fleeting advantageous environmental conditions and to overwhelm the abundant predators and parasites that can attack them.
Each individual of these 3 generations are genetically alike. Clones might be a disadvantage in a changing environment and pressure from pesticide applications since there is no recombination of genes afforded by sex. But insect ecologists have noted that spontaneous mutations in these clones are possible. If a favorable mutation occurs, then their populations are advanced quickly.
Most aphid species overwinter in the egg stage, and these eggs hatch in the spring into females that produce live young as described above. Several generations like this may be produced during the season. Sometimes winged individuals (see image below) may be produced that migrate off the original host plant species to a different plant species. In the latter part of the season, the aphids migrate back to the original host plant species and a generation consisting of both males and females are produced. The individuals of this generation mate, and the females lay eggs, which overwinter.
Aphid identification: Aphids have soft pear-shaped bodies with long legs and antennae, and may be green, yellow, brown, red, or black depending on the species and the plants they feed on. A few species appear waxy or woolly. Their feeding can cause distorted stems and leaves. They can transmit viruses. They often produce abundant sticky honeydew from their anus that can become colonized by “sooty mold” and become unsightly. Most species have a pair of tube-like structures called cornicles projecting backward out of the hind end of their body (see image below). Cornicles are like the exhaust pipes at the tail end of a hot rod, but in this case, secrete defensive fluid. The presence of cornicles distinguishes aphids from all other insects.
Davis G.K. 2012. Cyclical parthenogenesis and viviparity in aphids as evolutionary novelties. J. Exp. Zool. 318B:448–459.
Dixon A.F.G. 1985. Structure of aphid populations. Ann. Rev. Entomol. 30:155-74
Flint, M.L. 2013. Aphids. Pest Notes. Publication 7404. University of California Statewide Integrated Pest Management Program.
Gray mold caused by the fungus Botrytis cinerea is one of the more destructive plant pathogens, and it attacks a wide variety of plants. It is a common springtime disease, favored by cool rainy periods and high humidity. Conditions just like we have seen in California recently. Under these conditions, the fungus may sporulate on infected tissues and produce masses of characteristic gray or brownish spores that become airborne and spread.
Spores must have moisture—from rainfall, morning dew, or irrigation-- to germinate and infect plant tissue. Flower petals and ripening fruits and vegetables are particularly susceptible to infection. Botrytis can directly attack the flowers of roses and many other cut flowers in the greenhouse, and it can even develop, although slowly, at refrigeration temperatures during shipment.
Young, weak, or dead leaves and stems may become infected. Even seeds and seedlings may be attacked and killed. It often limits the productive life of flowers, vegetables, and fruit at the end of their growing cycle in the fall.
Once healthy flower petals or dead, dying, and damaged stem and leaf tissues are colonized, this diseased tissue can be used as a launching pad for the fungus to develop into mature healthy tissue. For example, under conducive environmental conditions, a necrotic tuberous begonia flower can become colonized with Botrytis and drop onto a perfectly green and healthy mature leaf and begin a new infection that can colonize the leaf.
Another example is when Botrytis colonizes a broken or pruned stem, and then Botrytis develops into the attached mature stem, where it normally could not directly infect. In this example with poinsettia, a broken leaf left a stub of dying tissue that became infected, and this stub became the launching pad for the Botrytis to enter the main stem. This led to a serious stem canker and blight, and all plants infected this way needed to be rogued from the crop.
Another example is seen below where annual statice (Limonium sinuatum) was grown for cut flower production. The plants were densely grown in a greenhouse with chronic high humidity and poor air circulation. When flowers were cut, long stubs were left behind near the crown of the plant. The cut stubs became infected and then moved into the young petioles of nearby leaves.
Spores may readily develop in decaying vegetation and old flowers, so removing old flowers before they become infected and function as spore sources or launching pads for infection of nearby healthy tissue is important. Have good air circulation in greenhouses to reduce condensation and wetness. See https://ucanr.edu/blogs/blogcore/postdetail.cfm?postnum=28597
Botrytis can form black hardened resting structures called sclerotia in decaying plant matter, which allows for survival during adverse conditions; so remove and dispose fallen leaves and debris around plants. Prune out any dying tissue.
Avoid overhead watering; drip irrigation or hand watering is best to keep water off flowers and foliage. Irrigate early in the day so that the foliage can dry as rapidly as possible. Maximize the period between irrigations to further enhance drying of foliage and flowers. There are many fungicides that help protect against infection. They can be used during periods that are conducive to infection. Be careful to rotate fungicide classes because Botrytis cinerea is known for its ability to mutate to resistant strains.
The previous blog demonstrated that container organic and inorganic physical components have limited nutrient holding capacity, and therefore the soil mixes they compose must be supplemented with nutrients during crop growth. The selection of the principal components of the soil mix therefore should concentrate on optimizing total porosity, water-holding capacity, air-filled porosity, and overall weight. Nutrients will need to be supplied continuously with controlled-release fertilizers and/or liquid feed.
Previous posts have generalized that relatively coarse particles favor formation of large pores in containers that increase drainage and air-filled porosity. Fine particles form small pores that increase water holding capacity. This generalization is appropriate for materials that have a very narrow particle size range. In most container mixes, however, accurate predictions of the air-filled porosity and water holding capacity are difficult or impossible because of the irregular shapes and varied internal porosities of the particles in the medium. The ultimate packing configuration of the particles is not easily predicted. The mixing practices, filling and packing of the pots, and settling of the media after irrigations impact the ultimate physical properties of the soil mix too. This, of course, is why soil mix physical properties should be measured with standardized procedures. Here are examples of properties of three container mixes measured under standardized conditions. Fig 1.
Consider using components based on their availability, uniformity, cost, stability, nutrient holding capacity, and freedom from toxicity, salts, pests and pathogens. Here are some common components that provide the principal structural framework of soil mixes and their properties.
Figures and tables adapted from: Management of Container Media by Richard Evans, UC Davis, for the class, ENH 120. Thanks to Richard.