Growth, Independence & (Mycelium R&D) Opportunity

I started my journey into mycelium technology from the vantage point of being a visual artist. The critical organizing principle I operated on as an artist was the belief in the power of practicing with intent along the gradient from naive to expert; not waiting until expertise resolved itself before taking my practice with a given subject or skill seriously (a principle of the ‘post-medium’, but we won’t discuss that here). Along this gradient of learning, at a certain point of critical knowledge mass (which occurs fairly early in the learning process), there is enough inertia to begin to behave and create novelly (to actually start making things with novelty and utility). This inflection point is powerful, and very often is more productive and impactful than achieving expertise itself. This critical inflection point along my journey in mycelium R&D coincided with developing an understanding of the criticality of the independence of specific growth rate and growth velocity. 

Orthogonal Dimensions of Physicality

When two dimensions are independent, it means that variation in one does not predict or constrain variation in the other (i.e. they are orthogonal). In practical terms, changing one variable does not force a change in the other; they can be tuned separately, and their combined effects can create a broader design space.

David Moore reconciles these critical concepts brilliantly in his work 21st Century Guidebook to Fungi. Fungi grow in filamentous form, meaning they expand by extending hyphal tips and by producing biomass (new hyphal material). It is crucial to distinguish growth from extension; specific growth rate (µ) refers to how fast the fungal biomass increases, whereas growth velocity (v) refers to how fast the colony expands outward (the linear extension rate of the colony radius). Specific growth rate is about making more mycelial mass, and growth velocity is about how fast the mycelial colony spreads over a surface or through a volume. These two are related but not the same; a mycelial colony can rapidly produce biomass without spreading quickly, or vice versa, depending on how it allocates growth into branching vs. tip extension. This decoupling is a unique feature of filamentous growth and is largely governed by the concept of the hyphal growth unit. 

The hyphal growth unit (HGU) is the average length of hypha (or volume of cytoplasm) per active hyphal tip in the mycelium. It is the total hyphal length divided by the number of hyphal tips. The hyphal growth unit represents the amount of resources supporting each growing tip. A larger hyphal growth unit means each tip has more backing biomass (or cytoplasm) and can potentially extend faster, whereas a smaller value means tips are more numerous relative to biomass. This parameter is not fixed, it can change during colony development. After growth initiates, the hyphal growth unit often increases initially (as the first hypha extends before branching much), then as branching kicks in it may oscillate and eventually stabilize to roughly a constant value once the colony establishes a steady state of growth and branching.

Specific growth rate and growth velocity are interrelated, but they can vary independently if the hyphal growth unit is not constant or if other factors modify colonial expansion independent of the underlying specific growth rate. In an ideal scenario where the hyphal growth unit remains fixed, growth rate and velocity are proportional; speeding up biomass production directly speeds up colony expansion. However, fungi can modulate the hyphal growth unit based on variability in branching frequency. 

For example, if the colony encounters a situation where tip extension is hindered, it can continue to produce biomass and will channel that extra biomass into making new branches (increasing tip number) rather than extending length per tip. In this case, colony expansion (velocity) slows down while biomass accumulation continues. Conversely, if conditions favor extension, the fungus may extend with fewer branches, increasing the colony margin even if the total biomass growth rate hasn’t changed. This demonstrates an inherent independence: a colony can expand at a certain linear rate (i.e. distance from colony origin to margin) while its total biomass increases at a different rate.

Density as an Emergent Quantity

When specific growth rate (µ) and growth velocity (v) are treated as separate dimensions rather than a single idea of “growth,” we can derive a clearer spatial logic. Each rate influences a different volumetric expression of the colony. Specific growth rate determines how much biomass the fungus actually produces; its skeletal volume. Growth velocity determines how quickly the colony occupies new space; its envelope volume.

The relationship between these two volumes defines the effective density of the colony:

Density = Skeletal Volume ÷ Envelope Volume

This framing makes several patterns visible that are easy to overlook when growth is viewed through a single lens. At a v and a µ, the colony extends rapidly but produces relatively little biomass to fill that expanding envelope. The result is a broad, low-density network: high envelope volume, low skeletal volume.

If v decreases while µ remains constant, the total biomass produced does not change, but the space in which that biomass is distributed becomes smaller. Density increases, not because the fungus is making more material, but because its spatial expansion has slowed. The same skeletal volume is now expressed within a reduced envelope.

It’s simple on its face, but a critical realization: density is the downstream consequence of how µ and v are expressed relative to one another.

A fungus can produce a dense, consolidated architecture simply by reducing envelope growth at a given biomass production rate. Conversely, it can generate sparse, exploratory architecture by increasing envelope expansion without proportionally increasing biomass accumulation.

The practical implication is straightforward but powerful; density, and by extension many material and morphological outcomes, is accessible through modulation of the ratio between µ and v. Fundamentally, these terms interact to shape the colony as a physical object.

Independence as an Expression of Agency

The independence between specific growth rate and growth velocity is a window into fungal agency. It distills the principle that a fungus does not grow according to a single, immutable rule. Instead, it allocates effort and chooses, within the constraints of environment and physiology, how to express its growth: as biomass production, as spatial expansion, or as some balance between the two. A doubling in biomass does not require a doubling in radial expansion, nor does rapid expansion necessitate substantial biomass investment. The organism can shift emphasis and privilege reach over consolidation, or consolidation over reach. This capacity is a form of agency.

Agency here should not be read anthropomorphically, it is not intention. It is the organism’s capacity to resolve environmental information into differentiated physical outcomes. Under one condition a fungus may allocate growth toward envelope expansion without building much interior mass. Under another condition the same genet may behave differently, routing biomass into skeletal elaboration and reducing spatial spread. Both outcomes are coherent, internally logical responses derived from the fungus’s rule set. The organism can produce different physical solutions from the same underlying metabolic potential.

The independence between specific growth rate and growth velocity is, therefore, more than kinetics, it is an expression of degrees of freedom in how it becomes itself through space. Each dimension describes a potential axis of response, and the organism can modulate either axis without proportionally affecting the other. In practice, this means the fungus is not locked into one architectural future, but possesses a distributed repertoire and the capacity to select from that repertoire based on local context.

Independence as Tunability

On its face this points to a fundamental principle of fungal growth that is easy to overlook, but to a mycelium engineer concerned with extracting value and operable range from fungal colonial behavior, the independence between specific growth rate and growth velocity goes directly to the core of physical plasticity. When two dimensions of growth are orthogonal, each can shift without proportionally constraining the other. 

Independence = Tunability, in the sense that the organism has more than one valid route through its physical expression, and the engineer has more than one axis along which those expressions can be influenced.

Tunability here does not imply precise control or direct steering of morphology necessarily, but rather it reflects the idea that fungal outcomes are shaped by how the organism resolves its own degrees of freedom under a given set of conditions. Because specific growth rate and growth velocity respond differently to context, adjusting the context does not yield a single predictable response, but instead changes the space of available responses the organism may select from. Independence widens that response space; it broadens the organism’s repertoire.

This framing is what fundamentally and irreparably shifted my own practice. Recognizing these two rates as independent made it clear that fungal morphology is not a singular, predetermined trajectory but a family of possible trajectories, each accessible under different alignments of internal and external conditions. Understanding that the organism has room to maneuver, and weight the probabilities of certain maneuvers become more likely, is what transformed fungal physicality from something to be observed into something that could be worked with.

Deeply internalizing this independence is essential for appreciating the opportunity space in fungal physicality. It is the conceptual foundation for viewing mycelium as a practical design medium and structural vocabulary.