Why do transplants I purchase, even small ones, bolt within two or three weeks of planting, even if the weather is cool? I suspect that the commercial growers are starting the plants way too early and keeping them in conditions that prevent them from growing larger; by the time we bring them home to our garden, the plants are already nearing the end of their growing cycle. What should the smart shopper look for when purchasing cool season plant starts?

These seedlings may be small when you buy them and thus look like very young plants. Our winter gardens benefit from a phenomenon called “vernalization” that permits plants such as broccoli and cauliflower to delay blooming until they reach an edible size. When exposed to some stress prior to planting, they mature abnormally fast. Often the cause is obvious; when removed for transplanting, the root ball is a solid mass of roots. Prematurity may also be due to moisture stress (even one episode of letting seedlings dry out), heat, or too much light.

Examine transplants carefully before buying, and don’t accept ones that have any appearance of stress, such as roots coming out the bottom of the tray or pot. Break up and loosen heavily root-bound seedlings before putting them in the ground. If you can, have a look at the roots before purchase by pulling the seedling slightly out of its cell. You can avoid early bolting by planting more bolt-resistant varieties and planting at the right time. Download a planting by date chart at http://ceyolo.ucdavis.edu/files/53274.pdf, or pick one up at the Master Gardener table in the Davis Farmers Market.

When is a tree invasive, and when is it one that has habits (e.g. seed pods or flowers) that we just don’t like? There is a Chinese flame tree (Koelreuteria bipinnata) by my front sidewalk that drops seed pods that look like little paper lanterns. What a mess! What do I do about the “volunteers” that sprout”?

I know one of this genus is an Arboretum All-Star, but I wouldn’t plant it in my yard.

The term invasive applies to plants that multiply in a number of ways (runners, bountiful self-seeding, rhizomes and more) and are more harmful or troublesome, difficult to control and contain in a specific area. Dropping seed pods, as you suggest, is more of a “messy” tree issue. Chinese flame trees are very adaptable trees, taking on cold, heat, drought, wind and air pollution and are prized for their large, loose clusters of yellow flowers followed by fat, papery fruit capsules, resembling little Japanese lanterns.

The Sunset Western Garden Book lists the Chinese flame tree (Koelreuteria bipinnata) as a deep, noninvasive rooted tree that is good to plant under. But it does caution to “control self-sown seedlings.” Is it possible to put down some weed cloth to prohibit seeds from taking root? Mulching around the tree is also good for keeping seedlings from taking root. Then there’s just the gardener’s scourge of physically removing seed pods before they have an opportunity to take root.

The Arboretum All-Star is the Koelreuteria elegans, Formosan flame tree. It can be viewed in the East Asian Collection in the Arboretum.

I see semi-circle holes and some completely round holes in the leaves of my roses and four o’clock flowers. What is causing this? How do I get rid of the pest?

These holes and semicircles are caused by the leafcutter bee, Megachile spp. These bees are industrious pollinators and should be encouraged or at least tolerated, rather than discouraged . They cut leaves and carry the material back to the nests for lining.

Leafcutter bees are native bees. They are absolutely essential in pollinating many native and wild plants. The leafcutter bee can be recognized by the light colored bands on its abdomen; it is about the size of a honey bee, and darker in overall color. The work of creating a nest and of rearing falls to the female who may lay 35 to 40 eggs, creating one cell for each egg. This creates 4- to 8-inch nesting cells. Each cell will contain a mixture of nectar and pollen and one egg; the young bees emerge during the next season. These solitary bees (they don’t produce colonies) need a material that they can cut through and excavate easily. You may see what looks like coarse sawdust at the entrance of a tunnel. Because they want soft, rotted wood, they do not cause damage to homes or other wooden structures.

Leafcutter bees are not aggressive and will only sting when handled. Their sting is far less painful than that of a honeybee. A charming interpretive sign at the Sonoran Desert Museum in Tucson said “These bees won’t sting you. They are busy, working moms.”

Next month , from the Garden Doctor’s “compost pile”: cockroaches snacking on slug bait, and safe forms of sulfur fungicide. See you then.

— Send questions, addressed to the “Garden Doctor,” by email to [email protected], voice mail to (530) 666-8737 or regular mail to UCCE Master Gardeners, 70 Cottonwood St., Woodland, CA 95695. Be sure to include your contact information because any questions not answered in the Garden Doctor column will be answered with a phone call or email to you. You can request the Yolo Gardener newsletter delivered by email and learn more about the Master Gardener program in Yolo County at http://ceyolo.ucdavis.edu/Gardening_and_Master_Gardening. Stop by and chat with us on Saturdays at the Davis Farmers Market!

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These semicircular holes are caused by the leafcutter bee. Courtesy photo

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