Digging In - Advice on Getting a Lemon Tree to Fruit

By Scott Aker
Special to The Washington Post
Thursday, March 12, 2009

Q I have a three-foot lemon tree that I started from a seed about seven years ago. The leaves are luxurious, the trunk about an inch across. I keep it outdoors on the east side of my house. During the winter it is near a south window and fluorescent lights (for other house plants). I have fed it with a slow-release fertilizer once (19-6-12). It hasn't bloomed or fruited. What am I doing wrong?

A The plant may not be receiving enough sunlight, it may not have the cool temperatures needed for flowering, or it may be too young to flower.

Citrus trees require full sun outdoors and plenty of light when indoors October to April. The southern exposure during the winter is certainly helpful, but fluorescent lighting has little impact for most plants if it is more than 12 inches from the leaves. High-intensity discharge lamps such as high-pressure sodium or metal halide fixtures are much more effective.

During the summer, keep the tree in a location where it will get sunlight all day. The east side of your house will be in shadow in the afternoon.

Lemons, limes and oranges are subtropical plants, which means that they need a cool season and a warm season that are very distinct. Without cool temperatures for several weeks, your lemon tree will grow new leaves but is unlikely to bloom. An unheated, sunny room will provide the cool conditions your citrus needs over the winter. It is also important to keep it outdoors during the frost-free period of spring and fall so it is exposed to cool weather.

Finally, there's the age of the plant. In orchards, lemons are grafted onto rootstocks and may produce significant crops in four years. A seed-grown tree in a pot will probably take longer.

Assuming your tree is old enough to bloom, stress may trigger flowering. Farmers in Sicily get a second crop by withholding water for a period of time and then heavily irrigating and fertilizing their trees. This process stimulates new growth, new flowers and another crop of fruit.

We have wood fires three to four times a week and accumulate a lot of ash. Can I use it in my compost pile or anywhere in the back yard?

You can use it in the garden as long as the wood was not treated with any chemical preservative. Wood ash is strongly alkaline and may slow microbial action, so don't add it to your compost heap. Wood ash contains significant amounts of potassium, so it is a beneficial fertilizer. It also contains calcium carbonate, which is the predominant compound in lime, and will help to increase soil pH. Certain plants, such as peonies and lilacs, benefit from this increase in pH if your soil is acidic, as are most soils in our area. Conversely, wood ash should not be spread around azaleas or other plants that grow well only in acidic soil. And don't use wood ashes where you plan to grow potatoes, because it can raise the pH sufficiently to favor a disease called scab on the tubers.

Scott Aker is a horticulturist at the U.S. National Arboretum.

© 2009 The Washington Post Company