Q. When is the best time to prune my oakleaf hydrangea, and how should I go about it? It has gotten much bigger than I anticipated!

A. Oakleaf hydrangeas (Hydrangea quercifolia) bloom on wood that grew last season -- you may have heard it said that they "bloom on old wood." They are best pruned as soon as they finish flowering. They begin to produce next year's flower buds soon after they bloom; if you prune them after the buds have set, you will reduce or eliminate 2010's flowering.

The best approach is to remove the thickest, oldest stems at ground level, rather than shearing them. That way, you will remove the flowers on those stems but not all of the flowers. You may be able to keep it in bounds that way. Shearing tends to cause a thicket of twigs at the ends of the stems, which block sunlight from the interior of the plant and reduce air circulation through it. This can create favorable conditions for diseases to develop, and it is generally stressful for the shrub.

Pruning is not a substitute for proper plant selection. If you have to constantly prune a shrub (or tree) to keep it in bounds, you may have chosen the wrong plant for the site. It does a disservice to both of you -- It creates more work for you and the plant cannot grow to its genetic potential. Expect most oakleaf hydrangea cultivars to grow 6 to 10 feet tall and wide, if not slightly larger. There are dwarf cultivars of oakleaf hydrangea that may suit your needs better, including 'Pee Wee,' 'Little Honey,' 'Shannon,' 'Sikes Dwarf' and 'Summit.' Most of these are 3 to 4 feet tall with a slightly wider spread.

Q. I grew my first vegetable garden last summer, and things grew pretty well. My question is about the inoculant that the catalogs recommend using with beans. What does it do, and how important is it to use it? I prefer to garden organically if possible.

A. Beans are classified as legumes. Legumes are plants that have the unique ability to use nitrogen directly from the atmosphere to fuel their growth. This is possible because they have a symbiotic (mutually beneficial) relationship with rhizobia bacteria. These soil-dwelling bacteria infect the roots of leguminous plants to form nodules. The bacteria lack chlorophyll and are therefore unable to manufacture their own food via photosynthesis. However, the bacteria are able to break atmospheric nitrogen down into a form immediately usable by the plants. In return, the plants provide the carbohydrates and sugars the bacteria need to live.

Leguminosae is a huge family that includes many familiar plants -- all types of beans, peas, clovers and lentils; flowers such as baptisia, hyacinth bean, and sweet pea; and trees such as black locust, honeylocust, Kentucky coffee tree and redbud. If you stop and look at the flowers and fruits produced by these different plants, you can see the similarities. The fruits are usually flattened (sometimes rounded) pods that contain individual seeds.

The inoculant recommended for use with beans and peas ensures that the appropriate strain of rhizobia bacteria is present in your garden soil. Although there is a specific strain of rhizobia that is appropriate for each crop, many of the inoculants you purchase and apply are combination products that will work for both beans and peas.

Although your beans would still grow and produce without the inoculant, your yields will be higher and your plants more vigorous if you use it. Inoculant is nontoxic and compatible with organic production. Remember that bacteria are living organisms and that the inoculant has a shelf life of roughly a single growing season.

Doug Oster writes a blog, "Growing With Doug," exclusively at PG+, a members-only web site of the Pittsburgh Post-Gazette. Our introduction to PG+ gives you all the details.