Bio-drilling and Avocado

by Laurie Meadows

Created 06 June 2019
Last updated 07 June 2019


Anyone growing a few avocado trees may find at some point they become infected with the root-rot causing fungus Phytopthora. The disease is easily spread on garden tools, in potting mix, and on footwear.

Currently, the disease can be managed with a mix of measures - adding appropriate mulch, treatment with formulations of phosporic acid, improving aeration and biological activity in the soil surface, and by improving drainage.

Soils such as those developed over volcanic material, sands and similar may already be free draining and need no further work. But suburban homes are hardly bought on the basis the soil may be suitable for a few avocados! The same may be true when buying a farmlet. So your soil may be 'sub optimal' - wet and lacking aeration. Even if it is 'fairly good', usually draining well, there may still be high-rainfall periods when the soil becomes saturated and practically without oxygen. And Phytopthora spreads in soil water, so these conditions allow it to infect more roots. Treatment with phosphoric acid may not be practical for a non-commercial avocado grower, so the best that can be done is to improve soil organic content and soil structure, and to improve drainage.

This article is focused solely on a single element of drainage interventions -  'bio-drilling' - a cheap and easy method to improve drainage.

What is bio-drilling?

Bio-drilling is the practice of growing annual or biannual plants that have a long tap root so that once the plant dies the root leaves a channel in the soil for water to drain away. According to soil scientists, some tap-rooted plants are able to penetrate compacted soils, even to depth of several meters. Where plants with tap roots create vertical channels of various depths, fine-rooted species like grasses make horizontal channels in compacted soil, creating limited vertical drainage.

How effective is this practice?

There is not much scientific information outside of a 'broad-acre' commercial vegetable or annual crop context - and almost none in the context of avocado growing. According to scientists, tap-rooting species have the longest roots in relatively friable soils - where their services are not needed so much. Paradoxically, while water drains a way quickly in friable soils, deep vertical soil channels may help avocado access moisture in the lower soil horizon in times of drought. (This is my speculation, and may not be true.)

Bio-drilling may be very useful where lower soil horizons drain reasonably well, but the upper soil surface horizon has become clogged with excess organic matter, impeding drainage. This can happen when green mulch, leafy mulch, lawn clippings, or large amounts of chipped tree foliage and twigs have been applied. The volume reduces available oxygen, thus slowing decomposition, forming a wet and heavy topsoil layer. If the impeding organic matter is pulled aside (it can be reapplied in late spring or summer), tap-rooted plants can be sown under the canopy. They will quickly form a taproot that penetrates the wet layer, and when they are later sprayed out the dead roots will hopefully have created a network of effective drains.

Dense sowings of radish (in particular) might compress and compact the soil due to the lateral expansion of the thick upper vegetable root. This supposed effect may be confined to denser, clayey soils. And may be offset by the improved drainage, and by the spongy organic matter left by the dead root in the 'drill hole'. Again, there is little actual data on this consideration.

Bio-drilling is not a panacea for poor drainage. If there is nowhere for drainage water to go, low points still fill with water. If low points can't be drained away, then without the possibility reshaping the growing area (mounding or raised-gardening) there is little point in bio-drilling.

Which plants are best?

The plants should be annuals or biennials, grow quickly, be able to be sown in autumn, not cast too much shade (if planted around young establishing avocado trees), and be easy to kill with non-hormonal herbicides such as glyphosate.

In the context of planting under existing larger avocado trees where there may be a lot of shading, then shade tolerant species seem obligatory.

Many of the Apiaceae (Umbelliferae) family have tap roots. Commonly known members of this family includes parsley, carrot, fennel, caraway, celery, and the weed Queen Anne's Lace. The Brassicaceae (Brassica family) also includes members with tap roots - notably some cultivars of radish.

When to sow, when to remove

For avocado, it is probably best to sow once soil moisture has replenished after summer. Any time from late summer to early winter would likely suit.

The bio-drill plants should be sprayed out once a reasonable tap root has been formed, and before the tops are too tall. Winter soils dry out best when there is no ground cover, so the sooner they can be mowed and/or sprayed the better. Radish it fast to 'do the job' - seed to sufficient maturity is about 8 to 10 weeks. Experimentation is needed.

Proven bio-drilling plants

Daikon radish (Raphanus sativus)
Long rooted cultivars of daikon radish is recommended for its ability for the vegetable to form wide holes from 20 to 40 cms (8 to 16 inches) in the upper layer. As importantly, the long thin terminal root extends even further down to form narrow channels penetrating deeper into lower layers. Radish seed germinates and grows very quickly.Once killed, radish plant foliage decomposes rapidly, helping winter soil surfaces to dry out. On the negative side, radish won't tolerate constantly wet soils (neither will avocado!).

After getting their tops mowed off, Daikon radishes decompose from the inside out, leaving a channel for water to get deep into the soil.

We like that.#covercrops

— Chris Sayer (@pettyranch) May 21, 2019
Daikon cultivars vary in the length of the swollen 'vegetable' part of the root. I have found no information on total root length of different cultivars. According to a New Zealand seed seller (Kings Seeds) the F1 hybrid cultivar 'Minowase Long White' has a swollen vegetative root 40cm long and 6.5cm in diameter. It is counted as being mature in 55 days from sowing. Minowase Long White seed is also sold on trading boards, and if this is home-saved seed from an F1 planting the vegetative root length may be shorter than the F1 culrivar.

Possible bio-drilling plants

Angelica (Angelica archangelica)
A biennial member of the Apiaceae family grows broad glossy leaves in its first year. I don't know how much taproot develops in a year.

Fennel (Foeniculum vulgare)
Fennel grown in late summer will be at least 30 cm high by the end of autumn, and have a tap root of at least 25 cm. Fennel plants are ultimately very tall, but they will have made good tap roots long before the plants get too big. Florence fennel is a smaller variety of fennel, and it may be better suited to bio-drilling.

Salsify (Tragopogon porrifolius)
A member of the daisy family (Compositae), and sometimes called the 'vegetable oyster' for its edible taproot, this plant is a good candidate as a bio-drill.

Black salsify (Scorzonera hispanica)
Closely related to Salsify, biennial, and with a thicker and possibly longer root (up to a meter). Supposedly tolerant of some shade.

Blue Lupin (narrow-leaf lupin, annual lupin) (Lupinus angustifolius)
Blue lupin has a fairly short taproot of 10 - 26 cm. It is 7 - 14 mm at the top. Tap root length and thickness varies by cultivar. These plants are intolerant of acid soils, and don't produce a good taproot is heavy soils. They may be best used for drilling a top layer whose drainage is impeded by poorly decomposed vegetative matter.