Green manure: which to choose - phacelia, clover, or mustard?

TL;DR

Green manures are plants you sow to make your soil richer. Phacelia grows fast and attracts helpful insects. Clover binds nitrogen from the air. Mustard suppresses harmful fungi. Sow in autumn or spring, let grow 6-8 weeks, then work in. They cost almost nothing, work better than artificial fertilizer, and you feel good about it.

Why sow green manure?

Artificial fertilizer is easy, but after a few years your soil feels empty. Soil structure shrinks, microorganisms vanish, and you need more fertilizer for the same result. Green manure works differently: you sow a plant, let it grow, and work it in. In that short time it:

• Adds organic matter (humus)
• Binds nitrogen (some types)
• Holds together against erosion (root-bound)
• Breaks apart compacted layers (roots pulverize hard zones)
• Attracts beneficial insects

After 6-8 weeks you turn it in, and your soil is noticeably richer. We have used this method for years with excellent results.

Phacelia: fast growth for impatient gardeners

Phacelia (Phacelia tanacetifolia) is a purple flower that becomes a jungle in 5-6 weeks. It grows truly fast, making it perfect if you want quick soil improvement.

Benefits:

• Grows explosively fast (60 cm tall in two months)
• Attracts bees and hoverflies (natural pest control)
• Root system breaks apart compacted ground
• Cheap seed
• Dense coverage (suppresses weeds)

Drawbacks:

• Does NOT bind nitrogen (not ideal for nitrogen-poor soil)
• Must be worked in while green (turning yellow causes problems)
• Frost-sensitive (sow after frost risk)

When: Sow April-May or August-September. Work in around October or (if sown in May) in June. Allow 3-4 weeks before ploughing; it decomposes best when green in the soil.

Clover: the nitrogen-binder for poor soil

Clover (Trifolium and Medicago) has nodules on roots where bacteria "capture" nitrogen from the air and convert it in the soil. Garden with poor, yellowing plants? Clover is your friend.

Benefits:

• Binds nitrogen (20-150 kg per hectare, depending on type and conditions)
• Removes nitrogen deficiency without fertilizer
• Stays good a long time (you can overwinter it)
• Great food value for organic farms
• Pretty flower (bees included)

Drawbacks:

• Grows SLOWER than phacelia (8-12 weeks to maturity)
• Only binds nitrogen if correct bacteria are in your soil (usually present)
• Stays long in soil (harder to plough than phacelia)
• Can become weed-like if you are not careful

Which type:

• Red clover (Trifolium pratense): Classic, fast growth, good feed value
• White clover (Trifolium repens): Stays lower, fine for lawn zones
• Lucerne (Medicago sativa): Deeper roots, longer lasting, harder to plough

When: August-September. Overwinters (frost-hardy). Work in April or May, before your next growing season.

Mustard: the fungus-suppressor for troublesome soils

Mustard (Sinapis alba, white mustard) grows fast and contains substances that suppress certain soil fungi and pests. Last harvest hit by root rot or damping off? Mustard helps.

Benefits:

• Grows almost as fast as phacelia
• Naturally suppresses certain soil fungi
• Good root penetration (breaks hard layers)
• Does not attract as many pests as phacelia
• Seed dirt cheap

Drawbacks:

• Does NOT bind nitrogen
• Can become weedy in some areas (prevent seed production)
• Less attractive to insects than phacelia
• Frost-sensitive

When: April-May or August-September, like phacelia. Work in after 6-8 weeks.

Step-by-step

Step 1: Identify your soil problem

Look at last season's growth. Did your plants grow slowly? Were there many fungi? Was the soil hard and compacted?

• Slow growth, yellow leaves -> Clover
• Fungal disease -> Mustard
• Hard soil, quick result -> Phacelia
• All fine but better -> Phacelia

Step 2: Sow at the right time

March-April: Sow green manures for summer turnover August-September: Sow green manures for overwintering

Break up your bed finely. Seed does not need deep sowing (1-2 cm). Sow heavily (10-15 g per m2) to suppress weeds better.

Step 3: Let grow 6-8 weeks

Give your green manure water (especially first two weeks). No extra feeding needed. Cannot see the ground beneath it anymore? Perfect sign. Wait for:

• Phacelia/Mustard: 50-60 cm tall, full bloom
• Clover: Dense and sturdy

Step 4: Work underground

Chop the green manure finely (or cut and dig). Work into soil at once, not weeks later (it rots away). Make sure it goes deep (at least 20 cm). Then wait 2-4 weeks before sowing or planting again (decomposition process).

Mixes: more than one type

Some gardeners blend green manures (e.g., phacelia + white clover). Advantages:

• Phacelia grows fast, clover follows
• Phacelia attracts insects, clover adds nitrogen
• More diversity in root depth
• Better soil structure after working in

Mix recipe: 70% phacelia + 30% clover seed. Sow together, work in together.

Frequently asked questions

Can I let my green manure flower?

Yes, fine! In fact better. Flowering means more root mass and beneficial insects. One caveat: do NOT let mustard go fully to seed (it self-seeds next spring as a weed). Phacelia and clover may flower.

Do I need special green-manure soil?

No. Regular garden soil suffices. Green manures grow nearly anywhere. Exception: in very acid soil (pH less than 6) clover grows poorly.

How long before I can sow again after working in?

2-4 weeks. In that time plant remains rot and nitrogen (with clover) becomes available. Too-fast sowing risks poor germination from fermentation.

What if my green manure will not grow?

Check your seed (fresh seed, not old?). See if it gets rainwater first two weeks. Compacted soil stops growth - loosen it first.

Can I use green manure in pot or tub?

Yes! Fine. Pot must be at least 30 cm deep. Phacelia grows well in pots, even on balcony. Clover too, though roots shorter.

Is green-manure seed expensive?

Rock-bottom cheap. One kilogram of seed costs 5-10 euros and covers 50-100 m2. Fertilizer costs twice that and brings less benefit.

Discover your own garden design

At gardenworld.app you can upload your front yard and see how your planting thrives in improved soil. Plan your green-manure schedule and visualize the result on gardenworld.app.