Doing a bit of horticultural sleuthing

I love horticultural detective work.

Every year I get questions from readers and friends that challenge my investigative skills. This spring has been no exception. But over the years, if I look back at the questions asked, the vast majority of them are related to why plants don’t flower or fruit.

If you do a cursory look on the internet, the answers to these questions are quite broad and general and seem to center on poor soil fertility, the wrong fertilizer (generally too much nitrogen), or the lack of light.

In fact, though, the real reasons are much more complex. And as one of my academic researchers told me least week, “Sometimes we simply don’t have the answers.” Nonetheless, here are some that you may find helpful.

The most common question this spring has been “Why haven’t my rhododendrons flowered?” There are usually two reasons when this occurs. First, is that the spent flowers were not removed the previous year and the plant engaged all its reproductive energy into seed production instead of bud production. Another possibility is that in removing the spent flowers, or while pruning, the new buds were inadvertently removed.

Last week I was speaking with a rhody maven who came up with another explanation that I’d never considered. She reminded me that last summer was very wet and cool. This is not the kind of stressful weather that would force rhodies into a reproductive frenzy. And she noted that in years when there is a summer drought, she thinks there is more bud (and thus flower) production for the following spring. It’s a stress response.

Earlier, I mentioned improper pruning, another reason spring flowering shrubs fail to flower.

It’s very common on forsythia when someone decides to do a heavy pruning to shape this plant, but does it late in the summer. This essentially removes all the flower buds that have formed and would have bloomed for the following spring.

I noticed the same thing at a condominium complex where the residents complained that their lilacs never seemed to flower. After speaking with the management company, I realized that during the summer their landscape contractor sent a crew to the property to shape and trim back the shrubs. All the shrubs. Yup, every summer they removed the buds that would be the lilac flowers the following spring.

This is also common with wisteria. These woody vines can take years and years to mature to the point where they will flower. If they are incessantly pruned to shape them and keep them under control—and not pruned at the right spots—they may never flower.

Many roses are similar in that, if they are improperly pruned, they may throw out only a few buds. But on some varieties that should rebloom again later in the summer, if pruning is not done at the right time and at the right spot on the cane, the second flush or successive flushes of flowers are often aborted.

Then there are plants like the Montauk daisy where pruning at the right time can result in fuller plants with copious blooms. The trick here is to prune the plants heavily in mid spring. The result is shorter plants with stronger stems and nearly twice the number of flowers instead of tall, leggy stems with heavy flowers that fall over at the slightest wisp of wind or downpour. Do this pruning too late and you’ll get no buds at all.

Next are the iris and peonies that fail to flower. In the case of these plants, the failure to flower is usually because the rhizomes are planted too deep. Many gardeners consider these parts of the plant to be roots and the logic follows that roots need to be buried. But rhizomes are not roots, the roots actually grow from the rhizomes and when the rhizomes are planted too deep the root development is disturbed, as is hormone production that initiates flower production.

Planting depth (or better put, lack of depth) on both these plants is critical. Don’t get confused though. The Dutch type of iris are not rhizomes and do get planted deeper, but the bearded type of iris is planted just below the soil surface.

Oriental poppies are a bit tricky also, as they are grown from root sections that are planted somewhat deep. Too shallow and all you get is foliage. Not enough sun, you get only foliage.

There is another problem with peonies. The rhizomes are planted at the proper depth, the foliage and flower buds develop properly, but as soon as the flower buds begin to swell they get covered with a fuzzy-looking growth that quickly covers the entire bud that shortly falls off. This is usually a fungus infection caused by botrytis. In most cases, the fungus comes from compost on the soil. And with the right moisture and temperature, it attacks the buds and destroys them. Simple solution: Don’t mulch your peonies, especially with anything containing manure. If you must mulch these plants, do so only after the flowering is done. But they really don’t need mulch.

Interestingly, the failure to flower is rarely a problem with annuals as most are hybrids that don’t produce viable seed and are bred to put all their reproductive energy into flowers. The exceptions would be species that are not hybrids and are most notably found in some marigolds, petunias and cosmos. In these cases, flowering can be encouraged and extended by simply pinching off the finished flowers. This is where the age-old art of deadheading comes from. If the flowers can’t set seed, the plant is programmed to produce more flowers and thus, we hope, more seed.

Heat can also play a role in flowering and we see this most often in the vegetable garden. A perfect example is the tomato plant. From May through July most tomatoes are madly flowering so they can flood your table with fruits during August. But come August, when we can often get temperatures in the mid to upper 80s, the flowers may show up but won’t “set,” as most tomatoes can’t set fruit at these higher temps. But as soon as it cools down, and as long as the plant is healthy, flower production and bud set will start all over again, especially in the indeterminate varieties.

Remember your radishes, spinach and lettuce? What happens when it gets hot? They bolt, set seed and taste awful. Have you ever seen peas flowering in July?

As for light, too much or too little can cause flowering problems. This is most evident in chrysanthemums, which respond to daylight to initiate the formation of flower buds. They have what’s called a “long-day response.”

Well then, how do they get the mums sold in supermarkets in May to flower? They are forced to flower with artificial lighting to extend the day length in the greenhouses. Christmas cactus, Thanksgiving cactus, Easter cactus, poinsettia, night-blooming cereus—they all flower based on a day length response. Plants that don’t bloom based on day length are called day-length neutral.

Planting depth is also critical for spring flowering bulbs, such as tulips, daffodils and the minor bulbs. Planting depths for each is different and so are planting times. Planted too deep, too shallow or too early and the flowering is definitely affected. So read the instructions that come with the bulbs.

And anyone who has grown amaryllis knows how critical pot depth and a period of dormancy is in getting these indoor planted bulbs to bloom. Plant them too deep, no flowers. Keep them growing year-round without a six- to eight-week dormant period, no blooms.

Cold also plays a part in the flowering and fruiting of many plants.

Many citrus plants need a period of time with night temperatures at or near 50 degrees in order to form buds and flowers. Apple trees need a certain number of hours of cold temperatures during the winter or they won’t flower.

Then there was the hurricane that hit the East End in the late 1970s. It was all wind and salt spray off the ocean with less than a half an inch of rain. The wind and the rain either burned or tore the foliage off of most trees that August. That November, just about at Thanksgiving, many apple trees were flowering and there was little fruit the following summer.

I hope some of this detective work will prove fruitful (or vegetableful). Keep growing.