A mesh cage over a whole plant or a breathable bag over a single flower isolates a seed crop from foreign pollen, but a cage also excludes the insects the crop needs, so insect-pollinated crops must be pollinated by hand or by pollinators you place inside. Choose a mesh with openings smaller than the body of the smallest pollinator you must keep out, and use cloth that breathes because sealed plastic traps heat and moisture and kills flowers.
Part of Community Seed Bank Management
Caging and bagging are physical isolation methods that place a barrier between a seed crop’s flowers and any pollen that is not wanted.
Choose By How The Crop Is Pollinated
| Crop behaviour | Barrier | What supplies the pollen |
|---|---|---|
| Self-pollinating (tomato, pepper, lettuce) | Bag a flower cluster or the bud | The flower itself |
| Wind-pollinated (corn) | Bag the tassel and the ear shoot separately | You, by hand, from a chosen plant |
| Insect-pollinated, large flowers (squash, cucumber, melon) | Tape or bag the bud the evening before | You, by hand (Hand Pollination) |
| Insect-pollinated, many small flowers (brassicas, carrot, onion) | Cage the whole plant or plot | Pollinators placed inside, or alternating cages |
For bag materials and the step-by-step procedures see Bagging Flowers. This article covers the choice and the failure modes.
The Cage Must Exclude The Right Insects
Mesh opening size has to be smaller than the body width of the insect you want to keep out. Fine enough to stop a bee is not necessarily fine enough to stop a small fly, and a tight mesh also cuts airflow and shades the plants. Check the openings by testing the mesh with a live specimen of the insect, not by eye.
For insect-pollinated seed crops, the cage keeps out outside pollinators and holds in the pollinators you add. The carrier suggests houseflies for crops that need insect pollination. Flies work for small-flowered crops such as onion and carrot; large-flowered crops are easier to hand-pollinate. Bees released inside a small cage may die or fail to forage, so test on a small scale first.
Alternating Cages Where Cages Cannot Hold Insects
Where you cannot keep insects inside, cover the cage on alternate days and uncover it on the others for insects to work. This halves pollination while leaving a risk of cross-pollination on the open days, so it is a compromise for two varieties flowering together and not a way to guarantee purity.
Heat, Moisture And Wind
- Heat. Cloth over a plant raises the temperature beneath it in full sun. Use light, open fabrics (muslin, organza, spun polyester, tulle) and avoid plastic.
- Moisture. A wet bag stays wet and breeds fungus on the flower. Remove wet bags and re-bag when dry.
- Wind. A cage must be staked firmly; a collapsed cage crushes the plants and lets pollen in.
- Gaps. A gap at the ground lets crawling insects in. Weigh the edge down with soil or stones.
Remove Bags At The Right Time
Bag the bud before it opens, usually the evening before. Leave the bag on through flowering and until a small fruit shows that fertilisation has happened. Mark every bagged flower with a thread or tag, because an unmarked fruit cannot be told from the rest of the plant.
Worked Example: Staff Time
The Phase 2 guide gives 5 minutes per plant for initial bagging and 2 minutes for monitoring and removal. For 30 bagged tomato plants that is 30 × 5 = 150 minutes to bag and 30 × 2 = 60 minutes to monitor and remove, 210 minutes, or 3 hours 30 minutes, in total. These are the guide’s figures; time your own first batch before planning a larger one.
Common Mistakes
- Using plastic. Flowers abort and fungus grows within a day.
- Caging an insect-pollinated crop with nothing to pollinate it. The result is empty flowers.
- Bagging after the flower has opened. Foreign pollen may already be on the stigma.
- Forgetting the label. You cannot tell bagged flowers from open ones later.
What’s Next
- Learn next: Hand Pollination — the method that goes with most bagging
- Combine with: Timing Isolation — reduce the number of plants that need bagging