Sunday, May 29, 2011

If April showers bring May flowers, what do May showers bring?

Written for the Davis Enterprise, May 26, 2011
Click on any image for a larger version


Last May I wrote a column which began,

"What's up with this weather? ....
Below average temperatures every month ... by as much as 10 to 15 degrees. If we complete May without breaking 90 degrees, it will be the first time since 1971. Pleasant as that may be for gardeners, the weekly rain and cool temperatures have played havoc with plants and crops."

Well, deja vu.
For the record, we did finish May 2010 without breaking 90 degrees. So far this year, other than a brief spate of windy, dry, warm weather May 3 through 5, temperatures have been below average as well. Mild temperatures and high humidity lead to disease problems, and rain in March left many fruit trees nearly fruitless.

So we get questions ...
There are dead branches on my apple tree!

Fireblight on an apple tree.

Fireblight is a bacterial disease that attacks blossoms, young fruit, and tender shoots of apples, pears, and related trees and shrubs. It is specific to members of the pome branch of the rose family. Ornamental pears and crabapples, loquats and pyracantha can also be affected. Symptoms are rapid dieback of short shoot. The affected parts look burnt, as if by fire.

Fireblight infects within a temperature range of about 55 to 80 degrees and very high humidity, so normal hot dry weather stops the spread. But it can do a lot of damage in a short period of time. Prune out affected portions, cutting down into healthy green wood, and throw away the prunings. Contrary to older advice, it is not necessary to clean your pruning shears between cuts. The disease is transmitted by bees, and by spores during periods of very high humidity.

The tomato plant I bought a couple of weeks ago is not looking good. It is losing leaves and has black spots on most leaves (fungus?). I would think it is the cool weather and especially that crazy hail we had. The top part of the plant looks okay. Will it heal itself, or should I buy a new plant?

Early blight is a leaf-infecting fungus on tomato seedlings. It was most likely on the plant when you bought it. Normally we don't have to worry about it here, because the fungus can't sustain low humidity. Cold temperatures have slowed the growth of the seedling, hail has made points of entry on the leaves, and rain has continued to splash the fungus upward. It will likely outgrow the problem.

Why is there hardly any fruit on my apricot, plum, peach, or nectarine?

European honeybees are finicky workers. They won't fly when the weather is cold, windy, overcast or rainy. We had 14 days of rain in March when the stone fruits were flowering. So some varieties got good pollination, but most didn't.
Apricots were hard hit by brown rot fungus again this year. The disease attacks and kills blossoms and developing fruit, sometimes spreading into the branch. Plums, and peaches and nectarines, also show dieback and fruit loss due to brown rot. Prune out the dead twigs as best you can, and mark your calendar to do a dormant spray next winter. Brown rot is the major factor limiting success with apricots here.

What is causing the holes on ...

... the leaves of my plum and pluot?

Shot hole fungus on a plum

Shothole fungus causes small areas of the leaf to die, and then the dead tissue falls out, leaving a small hole. It looks as though something has been eating the leaf. Not very harmful, but it does increase over time. The dormant spray noted above will help prevent it.

... the petals of my roses?

Earwigs like to crawl up rose bushes and feed on the thrips that are hiding among the rose petals. That is helpful, but unfortunately they also like to eat rose petals. Earwigs can be trapped with any fragrant cooking oil, left out in shallow bowls.

Katydids have just hatched out, and can be found here and there feeding on soft green leaves and rose petals. Their damage is transitory, and they move on quickly, so control is not necessary (or feasible).

What is causing the spots on the leaves of (1) my citrus; (2) my persimmon?

Spots on persimmon caused by environmental stress.

If you want to impress people, tell them the leaves have oedema. That is Greek for "spots." On thick-leaved plants oedema occurs during conditions of high or low temperatures, low humidity, and wind, It is an environmental stress and is not harmful to the tree. In the summer oedema can make Eastern redbud leaves (Cercis canadensis) unattractive.

Why are the flower buds of my gardenia falling off without opening?
Cool weather. Gardenia wants even, warm temperatures all the time. The plants will initiate flower buds at any time of year, but if the nights are cooler than about 55 degrees the buds will drop without opening.

If I plant hot peppers near my sweet peppers, will it make my sweet peppers hot? or vice versa?
If I plant a lemon near my orange, will it make my orange sour?

No, and no. The only time cross-pollination in the garden affects the part of the plant that you eat is when you are eating the seeds. So certain types of corn need to be isolated from each other, because you are eating the seed; never plant sweet corn near field corn or popcorn, or many of the kernels will be very chewy!
Oranges may be sour because the tree isn't getting enough sun. More commonly it is because you are harvesting too early. Many citrus turn color before they turn sweet.
The heat in peppers is caused by capsaicin, which is present in varying quantities in hot peppers and not present in sweet peppers. Hot peppers vary in their heat due to genetics (the type and seed strain of the pepper), environment (hotter in hot climates), and watering (reducing water concentrates the flavor, including the heat). Most of the hot pepper varieties we grow are open-pollinated seed strains, so their genes are more variable than hybrids. If your "sweet" pepper turned out to be hot, it was mislabeled. If your hot peppers aren't hot enough, try watering less.

Cross-pollination in the garden can affect the seed you save. If you carefully save seed from your favorite squash plant, but had a pumpkin nearby, you can get some very strange results in the next generation.

What are these dragon-looking bugs on my salvia?!?
Those are the larvae of ladybird beetles, aka ladybugs. The larvae eat aphids even more voraciously than their parents. You will also find an intermediate stage between the larva and the adult. Other beneficial aphid-eaters in the garden right now include leatherwing beetles and the larvae of lacewings. It is rare for us to need to spray for aphids once we have a resident population of beneficial insects. Sprays often kill the beneficials. So let nature run its course, and learn to recognize the good bugs.

Ladybird beetle (ladybug) larva.

Ladybird beetle pupa

Ladybird beetle adult

All the stages!

© 2010 Don Shor, Redwood Barn Nursery, Inc., 1607 Fifth Street, Davis, Ca 95616

Feel free to copy and distribute this article with attribution to this author.

Wednesday, May 4, 2011


What's that purple flower?!?

(Click on any image for a larger version)

You'd be surprised how often I get that question. In winter, it's rosemary or lilac vine (Hardenbergia). In spring, California lilac (Ceanothus) blooming in the Arboretum draws admiring visitors and bees. Thanks to the City of Davis, some of our roadsides have low-growing shrubs with vivid purple flowers right now: Spanish lavender (Lavandula stoechas Otto Quast).

Lavandula stoechas 'Otto Quast' in mid-street planting

Otto Quast is one of the Arboretum All-Stars, and for good reason. Lavenders in general are tough, drought-tolerant shrubs. What sets Spanish lavender apart is that it blooms early, often starting in February and continuing into the summer. The shrub has a low spreading habit.

Unlike other lavenders, it also has a set of showy petals atop the bloom spike. These act as a 'come hither' for pollinators, so Spanish lavender is less fragrant than other species. Breeders have introduced new forms with stronger or softer colors and some have even longer petals than Otto Quast, which will probably supercede it in the nursery trade.

What do lavenders want?

Lavenders are from the Mediterranean, so they are perfectly adapted to our climate of hot sunny days and dry summers. Growers in areas with summer rainfall struggle to keep them alive because of their sensitivity to crown rot. Commercial lavender production has been tried in many parts of the U.S., and the limiting factors are winter cold and high summer humidity, neither of which is an issue here.

Lavender farmers are advised to take extensive measures to improve soil drainage. From one publication: “Beds must be worked down 18 to 24 inches. It is best to raise the bed about 6 inches above ground level and mix in 1/3 sand, 1/3 loam, and 1/3 clay soil.” [ATTRA*]

A mixed bed of lavenders in June. The English lavenders are in full bloom, and the lavandins (hybrid lavenders) are just beginning to spike. Blooms will continue through the summer, even into the fall. Lavender plants like full sun and dry soil.

Well, folks, add one more advantage to living in California. We don't have to do any of that to grow lavender here. Just plant the shrubs elevated so water never stands around the stem: a raised bed is fine, but a slight mound or furrow is fine. Water should drain away from the plant. Water every few days to get the plants established. Then lavenders can be irrigated deeply and very infrequently.

I water my mixed lavender planting every 3 to 4 weeks with a drip irrigation system that runs for several hours or overnight. Established plants can survive the summer without any watering, though that may truncate the bloom season. In any case, never water them more than once a week unless they are in containers.

Gardeners get perplexed about what to do with lavender shrubs after a few years. The nature of the plant is that it sprawls and tends to lose foliage in the center. Light trimming with shears after bloom can encourage a bushier growth habit. Or you can just let them slowly fall apart, try cutting them back hard after a few years (sometimes this kills the shrub), or simply replace them. A new plant will bloom in the second year; often in the first.

In sum: lavenders love sun, tolerate extreme heat and wind, and prefer dry soil. There are a lot of different kinds!

Which smell best?

Traditional lavender for sachets and potpourri is English lavender (L. angustifolia). Hybrids between English and Portuguese lavender (Lavandula spicata) are called lavandins, and they are the oil producers. (Just for the record, English lavender is not native to England; it came from northern Spain.)

There are several chemicals in lavender flowers that make up the aroma, and the exact constituents vary between species, as well as what part of the plant contains the most of each. Portuguese lavender contains more camphor than English does. Some varieties have aroma compounds in the leaves, others in the flowers. French lavender has especially fragrant leaves.

The English flowers are sweeter, the others more acrid but with longer-lasting scent. So lavender products often contain both. Spanish lavender smells slightly sharp, sort of like turpentine, but the oil is used in air fresheners and insecticides.

Which look best?

Most have grey-green leaves. Some are nearly silver, and a few are variegated. French lavender has toothed leaves (dentate), and the Arboretum All-Star hybrid called Goodwin Creek inherited those. A number of varieties are more compact growers, making them better for small gardens.

When do they bloom?

English and hybrid lavenders start blooming later than Spanish does, mostly beginning in May. Lavandins bloom for much longer than most references describe, making them some of our best garden choices. I have found these often continue blooming into the fall.

A gallery of lavenders

click to enlarge

Lavandula stoechas

Lavandula stoechas 'Passione'

Lavandula officinalisEnglish lavender bloom spike

Lavandula pinnata

Lavandula x 'Grosso'

Lavandula 'Warburton'
has variegated, fragrant foliage

Bees love lavender flowers!

Here is a summary of the types,

including just a few of the many varieties available in nurseries:

Lavandula angustifolia ENGLISH LAVENDER

Bloom earlier than Lavandins; shorter spikes, darker flowers. The strongest sweet lavender fragrance. Very seasonal bloom (May - June here), and they often don't flower heavily until established.

o Hidcote Compact, very tight growth to 1-2 feet tall and wide, with darkest purple blue buds and flowers. Foliage varies from green to very gray.

o Lavender Lady Similar to Munstead, this seed-grown strain blooms the first year. The only lavender available in six-packs.

o Munstead-To 2 feet tall, spreading, with light purple flowers. Foliage usually green, but may be gray-green.

Lavandula dentata FRENCH LAVENDER

Big 4 x 6 shrub with green, toothed leaves (there is a gray version, L. dentata candicans) Showy purple flowers spring through fall. Foliage is fragrant but flowers aren't. Not common in the trade. Notable as a parent of the Arboretum All-Star:

o _L. x 'Goodwin Creek' -- 3 x 4 shrub with soft, densely woolly gray green leaves with coarsely serrate margins. Grown for great, gray leaves and attractive habit, with a continuous display of very dark purple flowers in spring, summer, and fall. Medicinal scent.

L. x intermedia LAVANDIN

Bloom later than English lavenders; longer flower spikes, fatter flowers. Originally grown for lavender oil. Later cultivars were selected for showiness. Flowers June through summer here, often into fall.

o du Provence To 3 feet tall, spreading wider, with lots of flowers over a long season. Very fragrant, very popular garden variety.

o Dutch Mill To 2 feet tall, 3 feet wide, with flower stalks to 18-24 inches long. Rich deep purple flowers are extremely fragrant.

o Grosso Compact growth with thick spikes of dark purple buds and purple flowers. Commercially grown for oil. Very fragrant. One of the best.

o Twickel Purple More violet-purple than Grosso. Very fragrant. Sometimes listed as a form of English lavender.

Two more are worth mentioning for their extraordinarily long bloom season, and frost tenderness. Often sold alongside hardy types, leading to consternation when winter arrives and they may be killed. Flower spikes are great for cutting, though they aren't fragrant. Worth growing for the abundance of bloom and intensity of color.


2 x 3 shrub with soft, ferny green foliage. Blue-violet flowers. Always in bloom. Tender; grown as annual here.

o L. pinnata buchii

3 to 4 feet tall and wide with feathery gray-green leaves and stalks of dark purple flowers which open spirally along the cluster through the summer. Tops are frozen back at 25 degrees; usually grown as annual here.

*National Sustainable Agriculture Information Service, ATTRA

For more information about the Arboretum All-Stars, visit their website, and see our Davis Enterprise article from December 2009.

Written for the Davis Enterprise, April 28, 2011

© 2010 Don Shor, Redwood Barn Nursery, Inc., 1607 Fifth Street, Davis, Ca 95616

Feel free to copy and distribute this article with attribution to this author.