Bulb Spacing Calculator

The bulb spacing calculator answers the question that comes up again and again when ordering bulbs: how many do I actually need? By entering the dimensions of your flower bed, the bulbs you want to plant and the desired arrangement, it calculates the number of bulbs required, the quantity to order allowing for wastage, as well as the resulting planting density.

This tool purposefully offers two different calculation methods. For rectangular flower beds a method is used where each position is counted individually, similar to measuring on your knees - this way the furthest away bulb flowers are also included in the result.

For areas that are circular, triangular, ellipsoidal or not fully contained within a grid, the calculation is based on area and density. The tool will indicate which method was used.

Two methods for calculating how many bulbs to plant:

A quick method is to use the number of bulbs per square foot published by bulb sellers and multiply it by the area of your flower bed. Many catalogs provide this calculation. For large-scale plantings with a loose arrangement, this method will suffice.

A more time-consuming method calculates the actual position of each individual bulb. For rectangular flowerbeds this is a simple arithmetic task but for a grid within a rectangle the results differ as there will be one bulb at either end of each row and along the edge of the flowerbed.

L and W are the length and width of your flower bed respectively, s is the distance between the centers of each bulb, r is the row spacing, b is the border spacing. The floor function means to round down to the nearest integer since you can't plant half bulbs.

These two additions are the only difference between the two methods. The effect is greater for small flower beds while it gradually becomes lost to rounding errors for larger ones.

For non-rectangular flowerbeds the calculator switches to density calculation. This means that one bulb is placed for every rectangular area with dimensions of s by r.

Square arrangement or staggered arrangement?

In the square arrangement, flower bulbs are placed at the corners of a square. This arrangement is easy to make with a string and it's simple to work between rows when weeding, so many people choose this method without thinking much about it.

In the staggered arrangement each second row is moved half of the distance sideways so that every bulb is placed between two bulbs in the next row. This makes three neighboring bulbs form an equilateral triangle. That's why gardeners call it a triangular arrangement, and also why the row spacing can be smaller than the spacing between bulbs in the same row.

If each tuber is exactly s away from the six surrounding tubers then the distance between rows is equal to the height of this triangle.

The gap between the rows is reduced by this factor to ensure that the distance between bulbs is always greater than or equal to s, which allows more bulbs to be placed on the same area.

This is the 15% that bulb suppliers always quote. It's not a rule of thumb but it's the most efficient way to pack circles on a plane and people stacking bees or oranges have come up with this same result long ago.

Offset planting makes the rows of plants almost invisible. When flowers bloom at the same time, they form a grid pattern of rows and columns that is especially noticeable with tall, single-stemmed blooms like tulips. An offset arrangement, on the other hand, gives the impression of continuous flower beds.

Planting distances depend on the type of bulb.

The spacing between plants depends on the size of the bulbs. Small bulbs are planted close together to create a carpet effect. Larger bulbs need more space so that they can store nutrients and grow again next year.

These are the values published by major bulb suppliers. The "bulbs per sq ft" figure is a simple guide used in catalogues and ignores edge effects, as it assumes staggered planting. So it does not exactly match the results of calculation tools.

If values are given as ranges, the denser range is for plants whose blooms will be admired only one season and then dug up, while the less dense range is for bulbs that should remain in the ground to multiply.

Choose a density to suit your desired effect.

Spacing and density are actually the same choice viewed from two perspectives. If you prioritize overall impression over individual numbers, use these fields and let a calculation tool figure out what spacing will achieve your desired effect.

Results will vary depending on soil type and sunlight. In well-drained soils with plenty of sun, plants can be planted closer together while in heavy, wet clay or where there is competition from tree roots you should choose the less dense areas.

Calculate the area of flowerbeds that are not rectangular in shape.

Most flower beds are not rectangular so this calculator supports four different shapes and also offers a more general method. All formulas are standard ones.

Some gardening guides simplify the calculation of an ellipse area to length x width x 0.8. The exact factor for an ellipse is π divided by four, which is approximately 0.785. So this simple method overestimates the area by about two percent. This tool uses the exact value.

Irregular flowerbeds:

For irregularly shaped flower beds, you can use the "strip method." Draw a line along the longest axis of the flower bed and mark equal-length sections on that line. Then measure the width of the flower bed at each mark.

Add up all the widths and multiply that result by the distance between marks. If enough marks are used, this error can be reduced to about five percent. This is a sufficient accuracy for ordering bulbs.

For example, a flower bed that is 60 feet long along its axis would be divided into six sections with each section being 10 feet. If the width were measured at five points and found to be 19, 22, 23, 19, and 13 feet respectively then the sum of 19 + 22 + 23 + 19 + 13 would be multiplied by 10 resulting in an area of 960 square feet.

A complete calculation example:

Consider a flower bed that is 12 feet long and 4 feet wide laid out on a square grid with common garden tulips planted at 5 inch intervals to the edge.

In the length direction, 12 feet is equivalent to 144 inches. If you divide 144 by 5, it will be 28.8, and after rounding off, it becomes 28. By adding plants on the outer edge, there can be 29 plants in each row.

In the cross direction, 4 feet is equal to 48 inches. If you divide 48 by 5, it will be 9.6 and rounding down gives us 9. By adding the outermost row, the flower bed needs a total of 10 rows.

If you plant 29 plants in each of ten rows, that will be a total of 290 plants. Since this is for a flower bed measuring 48 square feet, the number of plants per square foot would be just over 6. If you allow for a typical loss rate of 10%, then you should order 319 plants.

If you plant the same flower bed staggered, the distance between rows is reduced to 4.33 inches and 12 rows can be placed instead of 10, which results in a total of 348 bulbs. The size of the flower bed remains the same, and the spacing between plants does not change.

Planting depth:

A general rule for almost all bulb plants is to dig a hole twice to three times the depth of the bulb itself. When measuring the height, you should not measure from the top end but rather the bottom end of the bulb. So if your tulip has a two inch tall bulb then you would place the bottom end of the bulb in soil at a depth of 4 to 6 inches.

In heavy clay soil that drains poorly, plant the bulbs a little more shallowly since deep holes can lead to waterlogged roots. In loose sandy soil or where squirrels dig, you may want to plant them deeper. Deeper planting will delay emergence slightly which can be an advantage in low-lying areas with frequent late frosts.

Make sure that the bulb is planted correctly. The flat side should be facing down and the end with the sprouts facing up. If you are not sure which side goes up, simply plant it on its side. The bulb will then find the correct orientation by itself.

Plants in layers:

By varying the planting depth you can grow two or more layers of plants on the same surface area. If you plant tulips at a depth of 6 to 8 inches, hyacinths at a depth of 4 to 6 inches and crocuses at a depth of 2 to 3 inches - all in the same hole - you can enjoy blooms over an extended period of time in the same spot.

Move the individual layers around so that no tubers are directly above another one. If you use this method, you will need to run a separate calculator for each species because the distances between them vary from species to species.

Order: additional quantities, time frame, budget.

Order 5-10% more than the calculated amount. Bulbs can be damaged during transport and some of the bulbs delivered may be smaller. Also after planting it might look sparse in parts of the bed which will require a few extra bulbs to fill in.

Autumn flowering bulbs are planted when the soil temperature has dropped to about 50-60 degrees Fahrenheit. In most temperate climates this is usually between late September and November. Tulips are an exception, and it's best to plant them last of all. Wait until it gets cold enough before planting these.

If you are planting for the first time start with a smaller number of plants than recommended by the calculator. It is better to plant 100 plants carefully and complete successfully, than to plant 400 carelessly and give up.

Here's how to use this calculator.

Choose the shape of your flower bed and enter its dimensions. The flower bed input field has a unit toggle so you can do calculations in feet, meters, yards, centimeters or inches depending on what scale your measuring device shows.

Select a bulb from the list and load the recommended planting spacing or select the last item to enter any spacing. Then choose either square or staggered planting pattern. If you don't want to place your bulbs right up against the edge of the flower bed, then enter the distance from the edge.

When you open the order area, you can see additional options such as the percentage of extra quantity, an estimate cost, planting depth and how many bulbs you already own. The two charts below show the effects of your choice of layout and how dramatically the number of bulbs required increases when you decrease the spacing.

The spacing data given here is the median of published commercial bulb suppliers and is a guide for your planning but not a botanical recommendation. The optimum results will depend on variety, quality of bulbs, soil, climate and whether you want to dig up the bulbs after flowering so check the labels on the bulbs before ordering large quantities.