Small, lumpy strawberries? Pollination, cold or plant bugs

Small, lumpy or lopsided strawberries are most often a pollination problem, a cold-damaged flower or tarnished plant bug feeding, and the seeds on the surface show which. Seeds of mixed sizes mean the flower was pollinated unevenly; seeds that are all the same size, usually bunched at a hard tip, mean a bug. If your plant is giving no berries at all, that is a different question, covered in strawberry plants not fruiting.

Read the seeds first

What looks like a seed on a strawberry is a separate fruit, an achene. University of Delaware Extension says the receptacle may carry up to 500 ovules per berry, and that each one has to be pollinated for the flesh around it to swell. Skip some and the berry grows unevenly around them.

That is why the seeds are the clue. Delaware says poor pollination shows as variable seed size: large seeds were pollinated and small seeds were not. NC State Extension draws the same line from the other side, with uniform seed size on fruit damaged by tarnished plant bugs. Hold a bad berry up to the light and compare it with a good one before you change anything.

Patchy pollination

A strawberry flower has both male and female parts and can pollinate itself. Delaware says wind and rain can move pollen within the flower, but that this usually does not pollinate every ovule, and that bees are usually necessary for complete pollination. It gives 16-25 bee visits per flower as the recommended amount, and says this matters most for the king berry, the first flower to open on a truss.

Delaware lists the situations where it goes wrong: plants under row covers during bloom, and bloom periods that are rainy, stormy or cold. NC State Extension adds that poor pollination is very common in the first picking of spring-fruiting strawberries in the southeast, when the weather is often less suitable for bee activity.

For a pot, that gives you three things to check.

• Covers. Wisconsin Extension says to remove a floating row cover during bloom so pollinators can reach the flowers. A frost cover goes on at night and comes off in the morning.
• Access. A pot on a high or sheltered balcony may see few bees. The routine for moving pollen by hand is written for tomatoes and peppers in hand-pollinating on a balcony; strawberry flowers are a different shape, so use it as a picture of the problem rather than a method.
• Sprays. Minnesota Extension notes that bees are working the flowers at bloom and that even a certified organic spray is not automatically bee-friendly. Keep sprays off open blossoms.

Cold on the flowers

Strawberries flower early, and a late cold night can catch open blossoms. Delaware says frost commonly kills the whole flower, and that cold damage short of that still causes deformities because some achenes are damaged. It also says cold injury can cause fruit with multiple tips. The sign in the flower is the pistils in the centre turning black or brown.

NC State Extension puts numbers on it: with the flower completely open, 30 °F at plant level can cause some damage and 28 °F considerable damage. A berry that formed from a damaged flower stays deformed; protect the next flush rather than hoping the shape corrects itself. The steps for a cold night are in emergency cold snap protection.

Tarnished plant bugs

NC State Extension describes the adult as oval, flattened, about 6 mm (¼ inch) long and mottled brown, green and reddish-bronze. The nymphs are smaller and pale green, and the early ones look like green aphids but move faster. Wisconsin Extension points to a yellow V-shaped marking just behind the head to tell the adult from beneficial bugs.

Both stages feed on the flowers and young fruit. NC State says feeding on the developing seeds stops growth under them, leaving fruit that is small, knobby or unevenly developed, and in severe cases a hard, seedy tip with little flesh beneath. Wisconsin says the damage tends to sit at the tip, and that hollowed-out seeds are very characteristic. NC State also says the earliest feeding, at petal fall or before, causes the worst malformation. The common name for the result is catfacing; the tomato version has a different cause and is covered in catfacing tomatoes.

To check, Alabama Extension suggests laying a white cloth or sheet under the plant and shaking the plant onto it, where the insects are easy to see. Do it during bloom, before the damaged fruit appears.

What helps in a pot is mostly tidiness. Alabama Extension calls managing weeds in and around plantings one of the most effective cultural practices, since the bugs feed on many weed species, and NC State says they overwinter as adults in leaf litter and dead weeds. Clear litter and weeds around the pots. Wisconsin adds that earlier-flowering varieties may escape the worst of it. Damaged berries are still edible: Minnesota Extension says the fruit is less appealing for fresh eating but still edible.

Small berries that are not deformed

Not every small berry is a problem.

• Later sets are smaller. University of Maryland Extension says the first fruits are the largest and each succeeding set will be smaller.
• Heat and dry mix. Iowa State University Extension says container strawberries need consistent moisture because they have shallow roots and do not tolerate dry, hot conditions, and that fruit size tends to get smaller as soil temperature rises, especially for day-neutral varieties. Shade the pot rather than the plant; the options are in the heatwave guide.
• Age. NC State Extension says yield and berry size decrease after the first year and recommends replacing a planting after its second or third crop. Iowa State treats container plants as annuals, as set out in strawberries in containers.

A short check

1. Compare the seeds on a bad berry with a good one: mixed sizes or uniform.
2. Remove any cover during the day while flowers are open.
3. Look at the flower centres after a cold night for black pistils.
4. Shake the flowering stems over a white sheet for small, fast green nymphs.
5. Clear weeds and leaf litter near the pots.
6. Keep the mix evenly moist and the pot out of the hottest afternoon sun.

Sources

• University of Delaware Cooperative Extension, “Misshapen Strawberry Fruits, Cold Damage and Frost Injury”
• NC State Extension, “Tarnished Plant Bug in Strawberries”
• Alabama Cooperative Extension System, “Tarnished Plant Bug Damage in Strawberries”
• University of Wisconsin-Madison Extension, “Tarnished plant bug - a strawberry menace”
• University of Minnesota Extension, “Tarnished plant bug in strawberries”
• University of Maryland Extension, “Growing Strawberries in a Home Garden”
• NC State Extension Gardener Handbook, “Small Fruits”
• Iowa State University Extension, “How to Grow Strawberries in Containers”