Helen McKerralHow to improve flowering and fruit set

A recent snippet on GardenDrum got me to thinking about flowering and fruit set. Working in a nursery, I often see gardeners who are successfully promoting vegetative growth, but whose plants don’t flower or, if they do, don’t set fruit.

Medlar flower bud

Medlar flower bud

As the article above suggests, a host of complex biochemical interactions occur inside the plant to induce both flowering and fruit set but alas, a couple of university plant biochemistry subjects several decades ago don’t give me the depth of knowledge to discuss those complexities! However, in my experience, poor flowering and fruit set in the home garden are usually caused by just a few specific – and more straightforward – reasons.

1. No or few flower buds or flowers develop

There can be many reasons for this but, in my region and climate, three are the most common:

Medlar flower begins to open

Medlar flower begins to open

i. Too little sunlight/too much shade: Plants require a certain minimum number of sunlight hours to induce flower bud formation, and this amount varies with species, and even with different cultivars or varieties of the same species. And it’s not only the light provided by sunlight, but its heat that has an effect, which is why there can be differences depending on whether the sunlight is morning or afternoon, or in summer or winter. Day (or night) length also has an impact. Most garden books recommend that vegetables require a minimum of six hours direct sun daily, but it’s possible to harvest modest but worthwhile crops with less than that, though fewer flowers – and hence fruit – are produced. Of course, everything else – soil fertility, moisture etc. – needs to be ideal in this situation.

Medlar flower pre pollination

Medlar flower pre pollination

Shade can also be dense/solid, such as that cast by buildings and fences, or dappled/filtered, such as that cast by other plants. In summer when the sun is high, a certain amount of sun often reaches the southern side of fences, but the same site is shaded all winter. Conversely, you can often grow vegetables in winter below deciduous trees when they’re dormant, but not in summer when the trees are in full leaf and roots are actively sucking moisture and nutrients from the soil. It also varies with species – my red and white currant bushes and medlar flower and fruit happily in dappled shade without any direct sun at all, but so far the white sapote, while growing vigorously, has produced almost no blooms or fruit. Amount and seasonal changes in sunlight are essential considerations in the design of any productive garden.

Medlar flower after pollination

Medlar flower after pollination

Although flavour is less intense than when grown in sun, leaf crops such as lettuce, silverbeet, spinach, rhubarb and leafy herbs (chives, basil, dill, coriander, parsley, watercress) will often succeed in shade.

ii. Nutrient imbalances, notably too much nitrogen. Improving soil with plenty of organic manures and composts produces rich, friable soil… but you can end up with lush plants with plenty of vegetative growth, but no flowers.

Medlar fruit

Medlar fruit

I’ve always believed (and experienced) that potassium redresses the imbalance and stimulates flowering but this interesting article disagrees (the other pages on Dr Adam Dimech’s website are equally fascinating). Once again, the biochemistry is complex, although the author acknowledges that potassium may affect flowering indirectly by promoting overall plant health. If it’s a myth, it’s an all-pervasive one, still taught in universities and horticulture courses (which does not necessarily prove anything one way or the other!). I’d very much like to see the peer-reviewed, controlled scientific studies that show that potash has NO effect on flowering to which Dr Dimech refers – in the meantime, I’ll keep an open mind. The author does suggest that potassium can improve fruit set in some plants, and I have no doubt I’ve seen it improve my crops of bush currants and tomatoes. You can add potash in the form of soot, sulphate of potash, liquid fertilisers such as Thrive Flower and Fruit and Manutec Bloom Booster, and certain seaweed fertilisers or rock dusts; when buying the latter two, check the NPK ratio.

Bumper crop of eggplants

Bumper crop of eggplants

iii. Plant immaturity: Some fruit trees such as figs, peaches, nectarines and apples produce fruit very early, often bearing in the nursery when you purchase potted specimens, whereas others, such as pistachio, avocado and walnuts, need several seasons or longer to establish and develop cells that give rise to flower buds.

2. Flowers develop, but fruit does not.

Lack of sun, nutrient imbalances and plant immaturity can also limit fruit set, with three more common causes.

i. Lack of pollination: Lack of pollination is usually for two reasons. The first is a lack of bees. Perhaps flowers have appeared, but a subsequent cold spell has dropped the temperature to below 13-15 C, when most bees cease flying. If this happens regularly, you may need to choose a later-blooming cultivar.

Photo by orangeaurochs

Photo by orangeaurochs

If your garden lacks bees, you can encourage them by planting a wide variety of long-flowering, bee-attracting plants such as lavender, thyme, Nepeta, lemon balm, salvias and oregano, and plants that flower successively throughout the year so there’s always pollen available.

Lack of pollination may also be because your plant requires a cross-pollinator. Most almonds, cherries, plums, and apples require a cross-pollinator nearby, and even those which are promoted as self-fertile (eg All-in-One almond, macadamias) generally produce bigger crops near a compatible pollinator.

Avocado photo by avlxyz

Avocado photo by avlxyz

I’ve lost count of the number of nursery customers who have planted an avocado stone and nurtured the tree for decades without a single fruit. First, avocado cultivars don’t breed true so although you could be lucky, the resultant tree may also be one that produces few or no flowers (let alone fruit)… ever! Second, avocadoes need a cross-pollinator (‘A’ or ‘B’ type flowers). So when selecting fruit trees, do your research first, or buy them from a reputable nursery with knowledgeable staff, who can ensure you don’t waste years on a plant that will never fruit!

Sometimes, separate male and female flowers are produced on the same plant (zucchinis, pumpkins, squash). When bees aren’t doing their job, you can pick the male flowers and hand-pollinate the female ones, as this engaging video nicely demonstrates. Some gardeners advocate pollinating each female flower with two different male flowers; remember too that the female flowers are viable for only a day, so pollinate them daily, early in the morning when they first open.

ii. Irregular watering: Both flowers and tiny fruit may drop when plants dry out, even with just one missed watering. You don’t need to flood them, but regularity with a constancy of moisture levels is the key.

Eggplants already fruiting but the chilli plants in between are waiting for cooler weather

Eggplants fruiting prolifically but the chilli plants in between are waiting for cooler weather

iii. Temperature: Extreme heat can cause flower drop. Chillies are especially prone to this, I’ve discovered – it’s why I get some fruit early in the season, then nothing for ages, then, just as I’m beginning to think I’ll not get a decent crop, they come all at once at the end of the season when the hot spells are past. In my experience, chillies are more susceptible to this than eggplant or tomatoes, and I’ve noticed that even within chilli varieties there’s a difference. Next year, I’ll site my chillies so they get protection from late afternoon sun in summer.

Vegetables are grouped into warm, cool and mid-season crops, and this is often a guide not only of their preferred growing conditions, but the temperatures at which flowers are produced, open, and can be effectively pollinated, as well as when fruit will form.

A complex and fascinating subject, and I’d love to hear your experiences and opinions!

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Helen McKerral

About Helen McKerral

Horticultural journalist, photographer, contributor to many garden magazines, and author of 'Gardening on a Shoestring'. Adelaide Hills, South Australia

5 thoughts on “How to improve flowering and fruit set

  1. Hi Helen
    When I was rereading about phosphorus, I found it was really important in flower and fruit production. I was very surprised as I too had always thought it was potassium.

    • helen mck on said:

      Yes, it’s all very complex! The simplistic mantra is potassium for flowers and fruit, nitrogen for vegetative growth (leaves), phosphorous for roots… but perhaps this mantra is just too simplistic to be correct at all!

  2. Pat Hutchison on said:

    I got 8 bags of old sheep poo for Christmas! I like to use it as a mulch over the summer. I also decided to turn some of it into a liquid fertiliser for the veggies and herbs. So I was interested to read your views on potassium because not knowing too much about the value of sheep poo I googled it and discovered that it was not only a nitrogenous element but also contained potassium. Is that correct? Too early for me to form a view but my 35 year old cumquat (potted) already carrying fruit has displayed more blossom. It’s a mystery!

    • helen mckerral on said:

      Sheep poo for Christmas! You lucky thing – and I say that as a gardener, not sarcastically! To be honest, Pat, I wasn’t aware of that, but this document http://cetulare.ucanr.edu/files/82031.pdf appears to confirm that!

  3. Pat on said:

    Have to agree it is all very complex. All I can say is I follow the same routine each mid summer, Autumn, and Spring – after the sheep poo, comes
    cow manure, then some blood and bone and I use liquid Sulphate of Potash on anything that has a flower, except my natives. I also mulch heavily with cane and water well. Never a weed appears (wouldnt dare), plants grow strongly. However one plant that defies me is my Michelia Doltsopa which has never flowered, despite following all advice. Even gave her a good prune to show her who was boss. All she did was to promptly grow taller and now gets more sun which some advice suggested was the problem. I am still persisting with her. Gardening is such fun!!

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