Another article in the ‘Over Thinking Series’

Old growth coastal Douglas Fir forest biome, the Ponderosa Pine – Juniper Sagebrush ecotone, your meticulously cared for back garden and the neglected median strip running down a divided street all occupy our ‘landscape’ and include soil unique to their sites.  The soil type and structure is relatively easy to describe as it is defined by its physical properties…its biological components are considerably more complex and change over time in the long and short term as a landscape ages and/or suffers human disruption…and, can, in turn, affect some of the physical properties. (see:  The Biology of Soil Compaction.)

While not normally considered to be a component of soil, seeds are always there as part of the ‘matrix’ of life within it.  Seeds deposited by the plants growing on site, seeds brought in from other nearby sites by birds and other animals who either consumed them elsewhere and shat them out here or carried them in or on their bodies, seeds that were equipped with structures that allow them to be carried on the wind or to float down a stream.  These are the way seeds in native undisturbed sites move around and create relatively stable landscapes.  When a fire, slide or flood event happens, disrupting the growth on an otherwise undisturbed site, these are the seeds that are available to germinate and recolonize the landscape.  These seeds, stored on and in the soil, comprise the bank, or reserve.  Plants do not practice ‘spontaneous generation’.  There must be some form of propagule for growth to occur.  Whether acres are bared as a result of a hillside sliding away, one of the regular fires burns across a chaparral or a single tree toppled in a storm, these seeds, provide much of the germplasm to fill each particular niche.   Seed banks are the botanical store house, a physical and dynamic ‘insurance policy’ for each defined patch of earth.  Each seed arrives, time stamped, and is held until such a time that conditions support its germination.  If those conditions for germination are not met, the seed ‘waits’.  It’s capacity to wait is determined by the conditions of its ‘storage’ which include light, temperature and moisture.  There can also be requirements to break a dormancy period that are particular to the given seed and species which may include being ingested and so subjected to stomach acids, experiencing a ground level fire, going through a cold period of set duration, vernalization, or scarifying of the seed through some physical process damaging the protective outer seed coat.  Seeds are protected this way to help insure that they don’t all germinate simultaneously, or at the wrong time of the year, and may move away from the mother plant to help insure the overall survival of the population. 

Some seeds have little capacity to ‘wait’.  They must be sown immediately, be fresh, or their ability to germinate will be greatly reduced.  Some will quickly rot if conditions are not appropriate  when the ripe seed comes into contact with soil and only some of the conditions are supportive of growth, e.g., available moisture with cooler than ideal temperatures, think tomatoes and such,  ending any possibility for successful germination.  These, ‘failed’ seeds, may have been transported to an alien environment, alien for the plant.  Some seed will arrive on site that has no chance of maturing after germination because the conditions simply aren’t supportive.  Other seeds have the capacity to remain ‘viable’ over long periods of time, capable of being germinated, decades later.  There is a persistent myth, started by those seeking to cash in on the ‘Egyptomania’ craze of the 1800’s, that seed had survived 3,000 years buried in the Pyramids…, but this was never proven.  There has been seed demonstrated through radio-carbon dating methods that has survived as many as several thousand years cached away by rodents.  There are various institutional repositories, whose purpose are the long term storage of ‘germplasm’ to ensure the survival of various species and varieties that may be at risk for a variety of reasons.  On the ground, in the landscape, survival of seed for centuries and longer is relatively rare, because the natural forces work on individual seeds to break their dormancy beginning the process of germination.  (If you want to store a seed for a long period remember: cool, dry and dark…as in your refrigerator.  This does not mean that you could save any seed you might choose for 25 years and longer.  Each is unique.)  Once the process of germination is begun, the new plant either develops and grows or it spends what little energy it has and succumbs.  Having said that, the seeds of different species ability to survive in the landscape varies widely.

[A quick note here that not all seed produced is viable.  Some may appear normal but be sterile lacking essential elements of a seed, like the germplasm itself and this can be for a variety of reasons generally involving a lack of pollination.  Not every ovary in a flower may receive pollen.  Weather conditions must be agreeable for pollinators to be out doing their jobs.  Flowers could be damaged by weather or a stigma simply missed.  A seed may form with nothing within its seedcoat giving the impression that it is ‘good seed’.]

Any landscape’s seed bank contains seeds laying dormant on and in the ground, cached by squirrels and other rodents and the fresh seed still hanging on living plants.  Were an auditor to run a check on the ‘books’ at any given time in a landscape there are seeds laying dormant, germinating and rotting.  Seed accounts are being resupplied every year while others are degrading into inviability. All of this seed, generated from the site or from nearby, is appropriate in a natural stable landscape.  All of it could germinate and do well…when conditions are right.  Yet they don’t all germinate and grow.  While all of the seeds are undergoing very similar conditions, conditions that can break the seed’s inherent dormancy, not all seeds grow at once.  There is a built in variability to insure the long term survival of the landscape and each species.  Many seeds do germinate that will never reach maturity.  Some seedlings dry out as they are not capable of competing with their neighbors for limited water.  Others are not tolerant of the shade they find themselves growing in.  And still others may find themselves unable to compete for nutrients with the established flora.  Others yet may be consumed by various organisms or growing weakly, fall prey to disease.  In some cases it maybe that an established plant has an allopathic effect on its would be competitors.  If all seeds germinated each year and grew all of the surviving plants would be at a competitive disadvantage, all would be compromised by limited water, nutrients, light and physical space, they would be stressed and weak….This ‘system’, undisturbed, assures survival.

If the undisturbed native landscape that’s under no ‘pressure’ by users or adjacent landscapes is at one end of the continuum, the abandoned industrial, weed infested,  urban landscape with its topsoil removed, compacted and contaminated is at the other extreme.  There the native seed bank is gone.  Any native seed that may have remained has germinated and died or degraded to nothing over the years.  No plants remain to replenish the native seed bank.  The graded site has had its topsoil buried or spread elsewhere without the native cover to protect, replenish it or to provide the conditions necessary to maintain the tilth and soil biota many of those species require to grow with healthy vigor or to even survive.  On these sites are the ubiquitous weeds and the sometimes planted limited palette of typical landscape plants that have proven serviceable on poor sites.  Most of these ‘introduced plants’ survive only because of the maintenance and care provided by service companies staffed by people generally with a limited understanding and interest in the sites.  These sites are maintained utilizing an aesthetic that might be best characterized as neat and clean…minimal, at best.  These sites have seed banks that are ‘thin’, fragile and volatile and are comprised primarily of weedy species and aggressive seeders of a few of the neighboring landscape plants, e.g., non-native Maples, Oaks and Birch.

Over time the seed banks of these degraded sites develop a broader array of weedy species.  Depending on the frequency and thoroughness of the weed control program practiced.  Managers of these sites tend to place a low priority on landscape maintenance so the amount of seed banked in the soil increases with the vigor of weeds grown on site.  Opportunistic weeds move on to the site over time, diversifying the problem, via equipment, maintenance staff, amendments, mulches and contaminated plant material in addition to the natural avenues already described.  These grow, mature and ripen seed contributing to the bank.  Transportation corridors like roads and highways, rivers and railroad rights-of-way, also aid with the spread of weeds in ways described in an earlier post, (Landscape and Culture: Redefining the Urban Landscape – Single Use Corridors – Railroads.)  While the seed bank becomes more contaminated, very little desirable seed is accumulated in the bank, so it becomes ever more difficult to manage any given site.  Management strategies become ever more dependent on herbicide use and the planting of tough, utilitarian landscapes or simply maintaining them clean utilizing pre-emergent herbicides and sterilants where legal.  Weeds become firmly established in the soil and the bank assures that diligence will be required to keep the problem under control.  Many of our worst weeds have seeds that can persist in the seed bank remaining viable for years.  Clearing the slate of such a site by spraying it out once and then replanting may do little to effectively address the problem of the seed bank and can doom a new landscape to a very short ‘honeymoon’ period before it is again overwhelmed.  Just as a seed bank provides stability to an undisturbed native landscape it provides ‘stability’ to an out of balance weed infested landscape.

Most landscapes exist between the two extremes of the continuum, their health and vigor dependent upon our stewardship.  Contrived, built landscapes, are not somehow little modified versions of an undisturbed native landscape.  We cannot magically devise balanced landscapes on paper and install them with the appropriate relationships.  Some of us do our best to create landscapes more appropriate to the conditions on the site, right plant right place, while others insist on a particular look or plant selections that result in a high level of necessary ‘intervention’, maintenance, without a guarantee of success over time.  Such designs maybe ‘successful’ over the short term if the commitment is there and the seed bank is ‘free’ of unmanageable aggressive weed species.

As do other organisms, plants tend toward perpetuation of the species.  They grow, mature, flower and seed.

[Technically only flowering plants, Angiosperms, do this, but all plants, through seeds, spores and the growth of structures like rhizomes and stolons, tend toward increase.  Gymnosperms don’t flower they form male and female cones.  The pollen is released from the male cone, often carried higher on the tree, and is carried by the breeze to the female cone. Insects, hummingbirds etc. are never involved.  The seed forms on the upper side of a cone scale and is not contained by a fruit.  I know, Gingko, is an anomaly.]

Plants will normally contribute to the seed banks where they grow whether weed or desirable.  It is impossible in practice to prevent every weed on a site from flowering and producing seed or, intercepting and collecting every seed before it is distributed and deposited on the ground.  Some plants produce thousands of seeds individually so even the successful distribution of a small percentage of the seeds assures that the seed bank will become increasingly problematic over time.

The plants that we have selected for our gardens and landscapes tend to grow toward maturity, seed and produce plants to follow them, but…these don’t generally do this at a rate that is competitive with the weeds and so the proportion of total seed in the bank tends to tilt heavily toward the perpetuation of weed species.  Many selected varieties preferred by the landscape industry have been chosen, at least in part, because they produce little or no viable seed.  They may be sterile.  This came about because of the dominate landscape paradigm and its aesthetic of clean, static, neat, landscapes.  Weeds are weeds because they have continuously demonstrated their ability to re-generate at a highly successful rate across a multitude of sites, whereas selected landscape plants, in general, are chosen to assure their intended limited success.  This is largely the intent of landscape designers today.  A ‘wild’ dynamic landscape, capable of growth and change, is largely unwanted in designed landscapes, especially if they are urban.  Instead of supporting a dynamic healthy seed bank that is capable of responding to need in the landscape, we have attempted to nullify it, but have only succeeded in ‘breaking’ it.  By their nature weeds are more tolerant of both our cultural practices to control them and the herbicides that we choose to employ.  Seed bank ‘accounts’ then tend toward the success of weeds.

Negotiating a Better Weed to Desirable Seed Balance in Your Bank

If we are to be successful in our landscapes we need to better understand their complexity, including the role and operation of seed banks and adapt both our cultural practices and our aesthetic to do this.  We cannot continue to do what we are doing and naively expect the outcome to change.  There are two challenges we must address: we have to stop or significantly limit the deposit of weed seed into the bank; and, simultaneously, begin to grow the deposits we believe to be desirable and necessary.  Our landscapes are living biological systems.  Landscape soils are not sterile.  They are vibrant ‘partners’ in the life that manifests on their surfaces.  We must abandon our old model of the landscape as one that is nearly infinitely malleable and subject to our whim and fashion.  The land is not an inert background or substrate upon which we live and work.  This is a wrongheaded idea, part of the old paradigm, akin to manifest destiny on a worldwide botanical scale.  The old paradigm is an idea possible on only the smallest scale, where over time, we are committed to controlling all of the variables in a garden, knowing that this will require constant and perpetual vigilance on our parts.  This commitment is necessary for many types of gardens and the farming of food crops.  Even so, such gardeners and farmers must not forget what is at work on their sites and their work must be informed by this.

I have addressed the practice of weeding elsewhere.  I will not duplicate those efforts here.  Instead I will only say that we need to develop a new standard for health in the landscape, one not predicated on the identification of disease (here including weeds) and their control, an approach based on pathology, which is a negative approach, but on one that is positive, based on the idea of building health, that assumes health, vitality and vigor can build on itself.

Landscapes are living dynamic systems characterized by an active cycling of energy, resources and organisms.  Just as organisms tend toward reproducing themselves, they also age and die, all along changing their relationships with the other members and elements at play in the landscape.  Each species’ life cycle and span are unique to itself ranging from monocarpic annuals to a handful of conifers like Coast Redwood that can live for thousands of years.  To freeze a landscape in time, as if it were a painting, is to attempt to hold it separate from death, thus removing it from the natural cycling of all living things.  Seed banks are a vital piece of the landscape.  The modern landscape has been removed from the natural world and it is breaking down at an increasing rate.  Today’s landscapes are not the same as they were 20 years ago or even last year.  Our designs and methods have attempted to hold them out of play and their ‘historical’ buffer, their seed banks, have deteriorated and now play a negative role wherein they offer an increasingly negative ‘pressure’ actively degrading these static works of art we move through and live in.

If we choose to keep these static landscapes there will be ever more need to control the weed pressure by chemical means or to just walk away from them and give up.  Politically it is becoming more difficult to depend on the use of herbicides.  Even if it weren’t, the use of herbicides, particularly soil sterilants and pre-emergent chemicals intended to prevent the germination of weeds, with the additional effect of preventing germination of seedling volunteers from mature plants that were intended by the designers, creates a situation where only those weeds resistant to chemical control are able to replenish the seed bank pushing it toward an even more untenable balance, one dominated by seed more resistant to herbicides or more deeply insinuated within the remaining landscape plants.

If we move toward a more dynamic landscape model, one characterized by ‘adaptive management’ wherein we attempt to do the best job we can, we would install a landscape that is best suited to the conditions we can identify and then through a series of adjustments, tweaking our on the ground practices in a way that will ultimately move the site towards a healthy balance without adding to our management burden.  We stop doing the things that we see contribute to the problem on each site building healthy soil, reducing the weed load and tweaking the plant palette to create plant communities that reinforce themselves.  This cannot be drawn up on paper…only its starting place, the criteria by which we can perform an ongoing evaluation and the ultimate goal.  The seed bank is an integral part of the puzzle.  It will influence and in turn be influenced by every other piece.  What has been missing up until now is that its role hasn’t been recognized, nor has the dynamic nature of any landscape and the unpredictability of the urban landscape.  Fixed static landscapes belong to the world of landscape painting, of artists like Watteau and Constable, not the living landscapes of which we are a part.

Weed Viability

The following is a brief list of some of our common weeds and a few factoids addressing their seed production and their ability to survive in the seed bank.  It is important for the gardener/landscape steward to be aware of the problems on their site, to understand how their work may be moving them closer or further away from some kind of meaningful balance in the landscape and its seed bank.

Canada Thistle –  Cirisium arvense – can produce over 1,000 seeds per flowering stem each which can remain viable for 22 years in the soil, deep burial increases longevity

Old Man’s Beard – Clematis vitalba – can produce 100,000 seeds per sq meter of flowering ‘canopy’ that can remain viable for 5 years in the soil

Hairy Cat’s Ear or False Dandelion – Hypochaeris radicata – about 1,000 seeds per plant, with 60-70% germinating in the fall/winter.

Velvet Grass – Holcus lanatus – can produce at least 1,000 seeds per plant, does not survive burial, some portion has delayed germination.  95% remain viable after two years.  Surface seed needs to be germinated to control.

Scotch Broom – Cytisus scoparius – Can produce over 9,000 seeds per year that can remain viable for an estimated 30 – 80 years

Creeping Woodsorrel – Oxalis corniculata – Can produce 10-50 seeds per pod up to 5,000 per plant. It is unknown how long the seed can remain viable in the soil, but germination can go to zero on hot days with moisture.  Can germinate early spring through fall.

Himalayan Blackberry – Rubus armeniacus -Mature plants can produce 7,000 to 13,000 seeds per sq meter with 10 -30% germinating annually, most are shaded out beneath the plants. Seeds remain viable for several years.

What’s in Your Seed Bank Today?

Whatever the plot of land is that you find that you are responsible for, know that within and upon its soil is a bank of seed, seed that is genetically disposed to germinating and growing.  It would be the rare individual, or a very young one I suppose, who can trace their entire history to a single place, but a plant spends its entire life in one place…in nature, even whole generations.  Most of us become aware of place only over the years as we begin to reach out beyond the narcissistic imperative of youth.  Knowing it intimately is something else entirely.  The seed bank, in a sense, represents the memory of a landscape.  Deposited there are the mundane as well as seeds that promise dramatic change, changes that will occur if we do not pay attention.  Learn from it or suffer it…it’s our choice.

Gardeners inherit a very particular set of problems with a new landscapes and central to these are the seed bank therein.  There is never really a blank slate, even when said landscape has had all of its top soil horizons scraped away (O, the organic layer and A, the layer commonly referred to as topsoil).  Such sites have the huge handicap of severely compromised soil conditions.  A virtually blank seed bank in such cases simply buys us a little time, before the mistakes of our neighbors and the forces at play across the wider landscape begin to ‘even out’ the playing field.  If our landscape is small enough and the weed pressure is low enough, we can find our balance relatively easily, but when the demands are great we have to be timely and very thoughtful in our work decisions.  We have to practice good horticulture.

We have to limit the annual additions of new weed seed to our bank.  The weed seed already on site must not be allowed to germinate freely and grown on in an established landscape if we want the garden/ landscape to continue.  If we can do this, time is our ally, because all seed’s have a reduced ability to germinate as the years go by, remember that this could be as many as 80 years in some cases.  Vigilance and consistent action are mandatory. Our control tactics must be considered and specific.  A thick layer of mulch will block the needed light from those that require it for germination.  A thinner layer of mulch will only tend to moderate temperature and moisture levels, which may be to the advantage of other species’ germination.  Summer irrigation may provide the needed moisture to many Eurasian weeds, many of our most common and aggressive, so stopping would help limit weed seed germination.  The practice of surface cultivation may increase the numbers of seeds brought up to the surface with enough soil contact, moisture and light to better germinate while also loosening the soil and thus improving the tilth permitting quicker penetration by newly growing roots…, even though it may ‘kill’ young vulnerable weeds.  In general soil disturbance of any kind favors seed germination.  A better strategy would be to quickly establish a cover appropriate for the site to negate the need for cultivation, help shade out seed and to compete with water, nutrients and light that most germinating seedlings require.

Unbroken cover, which may include several layers from ground to canopy, is your best strategy for weed control, but it is not sufficient in itself.  Given the presence of invasives monitoring and regularly roguing will always be necessary.  Keep in mind that whatever tactic you employ for weed control there will be consequences that need to be considered, because each plant community will vary and successful weeds will select themselves over time that can best compete with the given conditions…including you and your tactics, your cultural practices, will be part of that selection process.  Adaptive management is key here.  Watch. Learn and Adapt.  The weeds will so we must as well.

You may decide to attempt to exhaust the weed seed in your bank.  Again, remember, that much of the worst seed can last for years.  Still, after weighing the many considerations, you could choose to germinate, spray out, shallowly till and repeat, to reduce the weed seed accounts and this will do that, but will it be enough?  Others may attempt to ‘solarize’ the soil to the same end.  Will the loss of desirable seed as well be acceptable?  Will the consequent reduction in soil health and tilth?  You need to think about how many years and which species of weeds have been adding to the balance over the years.  I’ve been successful, at a garden scale, with very few perennial rhizomatous or stoloniferous weeds, covering the soil with cardboard and burying that under 6″ of organic mulch for a year.  That’s what I did at my home garden and the only plants that were problematic were a particular aggressive Scilla the previous owners had established and nodding onions, both of which are still an issue 25 years later, though miner.  Very often we don’t feel we can afford such ‘delays’ in our work given expectations of our clients or of the public.  These are all choices intended to give the gardener or land manager a ‘leg up’.  They are not a cure.

If there is a cure it lays in a coordinated effort across the human community to change our view of and practices in the landscape.  It is necessary that we all start seeing ourselves in relationship with the landscape, not as a separate object, that our otherwise unconcerned lives play out across.  Humans are members of the same larger community, though many, if not most, have opted out of an engaged role.  A seed bank is an active player in the living landscape.  Trying to negate or remove it has brought all of us a lot of problems…problems that won’t go away.  In a sense a seed bank is like a physical object that is moving through time and space.  It has momentum.  We cannot expect to turn it easily in any direction we might choose.  It will take sustained effort and that effort will have to be consistent with the life inherent in the place.  I once read a book titled, The Poetics of the Garden, it spoke of harmony, fit…the ‘rightness’ of a design to a place, about the ‘genius’ of a place.  That was a landscape architecture classic, still referred to.  What I am talking about is a little different, but it is still about ‘genius’ of place.  If we watch carefully we will observe through the response of a place, a landscape, what its tendencies are, what ‘direction’, given its momentum, it ‘wants’ to go.  Place then can ‘show’ us the way, if we are patient and receptive.  There are a lot of ‘ifs’ in this path.  We are on our own here.  There is no book to show us how to fix our broken landscape.  Indeed, there will be many ways and none of them can be found through ‘formula’ or insistence.  It is a time for sensitivity, rigorous thought and creativity.