Phacelia Tanacetifolia – A.k.a. Green manure




As crops are harvested into late summer, early Autumn we are generally left with a number of empty vegetable beds over the winter months. It’s really important not to leave soil exposed as this can lead to soil erosion and the leaching of vital nutrients. Additionally, those pesky weeds are likely to emerge and rampage over the empty vegetable beds during the autumn and winter months, again removing much needed nutrition from the soil.

There are a number of ways exposed soil can be protected from weeds and the elements.

Cover the bare soil with some form of mulch. Mulch is basically anything that covers exposed ground, which may include cardboard, straw, wood bark, grass clippings, etc. We tend to use a heavy duty fabric membrane that we buy from the garden centre. However, this is an expensive option and we’ve found the strong coastal winds tend to whip up across the allotment during the winter months causing the membrane to be ripped from the pegs that secure it. Last year we had to resort to securing it with planks of wood, bags of compost and full watering cans,  just to prevent the membrane from taking off!

Using mulch is fairly effective. It keeps the ground weed free, although saying that, let me remind you about the bindweed roots that we found under some carpet when we took over the orchard plot!


Note: Never dig mulch into the ground. Organic matter is essential for fertile, productive, healthy soil. All organic matter takes time to breakdown, and all organic matter is broken down by micro-organisms. Fully decomposed organic matter is known as humus, it’s black and crumbly. It’s not possible for humus to decompose further. Humus stores the soils nutrients, moisture and helps improve the soils structure and drainage. Mulch on the other hand is organic matter that hasn’t decomposed. It is yet to go through that process. Therefore it has little to no benefit to plants if dug into the soil. The decaying organic matter may actually hamper vegetable growth rates as the busy micro-organisims use nitrogen and phosphorus and other nutrients, also required for the successful growth of vegetables.

Another option is to sow green manure. Green manure is essentially a fast growing plant used to cover bare soil. It helps improve the health of the soil. The foliage covers exposed soil helping to suppress weeds, and the plants root system helps prevent soil erosion.

There are a whole host of plants that can be used as green manure, and one should be mindful of the cultivar before sowing. For example, there are a number of plants such as, Alfalfa (Medicago sativa), Alsike clover (Trifolium hybridum), Bitter Blue lupin (Lupinus angustifolius), Crimson clover (Trifolium incarnatum), Fenugreek (Trigonella foenum-graecum), Trefoil (Medicago lupulina) and Winter field bean (Vicia faba) that all belong to the to the legume culivar. These plants are nitrogen fixers and this should be noted when considering crop rotation. 

Mustard (Sinapis alba) is part of the brassica cultivar; to avoid club root, this too should be considered when planning crop rotation. 

This is all rather complicated for my simple mind, so to keep things easy, especially with crop rotation in mind, we always tend to opt for a green manure that is neutral and that will not impact crop rotation.

We sow Phacelia (Phacelia tanacetifolia). 


Ultimately, we are looking for a green manure that helps suppress weeds and prevents soil erosion, phacelia ticks both boxes. It also has the added bonus of helping improve soil structure.

Phacelia, otherwise known as purple tansy, belongs to the Boraginaceae family. Other members of this family include, borage, comfrey, forget-me-not and heliotrope. 

Germination is aided by cooler temperatures and darkness. Sowing is recommended between, March to June or August through to October. Cast the seed at roughly 1 gram of seed per square metre and cover with roughly 1cm of soil. It likes most soil types providing there is reasonable drainage.

Sowing phacelia.

Prepare the soil by clearing as many weeds as you can

Water the soil

Cast the seed

Then either rake or as I do, gently fork over.

Water again.


The seeds take anywhere from 12 to 30 days to germinate. The ground should be kept damp throughout this period. The plants should be fully grown within 3 months.

Phacelia has a dense fern like foliage, perfect for suppressing weeds. Providing it’s not too cold, it should overwinter well.


In the spring, cut down the foliage before it flowers and dig into the soil. The advantage of this is that all the nutrients absorbed from the soil will be released back into the ground, plus the broken down matter will help improve the soil structure. Once dug into the soil, allow at least two weeks before planting or sowing seeds to enable the plant structure to break down sufficiently.

If allowed to flower, this plant will not only look spectacular, but it will hum with the activity of busy bees.



Posted in Green manure

Leave a Reply

Fill in your details below or click an icon to log in: Logo

You are commenting using your account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s

October 2014
%d bloggers like this: