Downsides:

 

1. decomposing woody biomass tends to cause the mound to collapse over time, making them unsuitable for planting perennials (soil eventually compacting around roots) [228]

2. it is easy to over-fertilize a hugelkultur bed [228]

3. works best under light water loads. Too much water could cause non-decomposed wood to become buoyant, destroying the mound and causing flooding [229]

4. the new mound provides lots of disturbed surface area easily colonized by opportunistic weeds [228]

5. woody biomass is less exposed to the elements and oxygen, slowing decomposition and potentially going anaerobic, acidic, and anti-microbial [230]

6. water retention works best after the wood has had time to decompose and spongify

7. excess carbon in the bed (from the wood) can cause nitrogen drawdown as the wood decomposes, resulting in nitrogen deficiency, especially in the first year. [233] Placing "green" nitrogen-rich biomass immediately adjacent to the wood in the mound can alleviate this problem.

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Note: do not use wood species that are alleopathic (black walnut), resist decay (cedar), green trees that sprout easily (willows), or treated woods (railroad ties). [231] Also avoid trees like firs and pines that can leach tannins into the soil and turn it acidic. The best woods to use are those like apple, alder, maple, oak, poplar, and acacia. [233]

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Variations of hugel construction can also be practiced based on available local materials, for example: replacing woody biomass with rice grass. [232]

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Hügelkultur beds are useful for planting annuals but should not be used as swales to direct and retain large volumes of rainwater. Their tendency to collapse over time and their subterranean architecture also make them poor choices as a soil base for fruit trees and perennials.

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As a method of raised bed construction they have their advantages and disadvantages, and may make more sense in some contexts than in others.

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