A feature by our award-winning National Collection holder, Sarah Cooke…
Wild tulips, by which I mean species tulips growing in the wild, have a wonderful mechanism to ensure that the bulbs grow at an ideal depth for the conditions in which they are found. This seems like magic, when you realise that they usually begin as seeds germinating on the surface of the soil. We have a good number of Tulipa sprengeri growing in the garden; mature, flowering sized bulbs are always found growing approximately 15 cm below the surface. From seed to flowering usually takes about four years. During this time they are busy ‘burying themselves’ and growing larger and stronger bulbs each year.
The ‘new’ bulb forms at a greater depth each year, by means of a ‘dropper’. A leaf axil (a bud) forms within the bulb, this is not a ‘normal bud’, but one which extends downwards as a tubular extension into the soil (see picture). A dropper is sent down each year until the desired depth is reached. If a bulb is accidentally disturbed and brought up in the soil, it will again send a dropper down to the correct depth. Tulip cultivars seem to have lost this ability, and if they are planted too shallow they split into three or more smaller bulbs, which rarely flower again. This is good for commercial bulb producers, as they can stimulate splitting by planting shallow and then planting the small bulbs deep to form good sized flowering sized bulbs for sale.
Tulpa sprengeri bulbs are thus rather expensive to buy, as they do not split and make flowering sized bulbs readily. The only way to produce large numbers is to sow seed, and wait, meanwhile keeping them weeded, etc. Fortunately for me, T. sprengeri like the conditions and protection given by iris rhizomes (which do not use much water in the summer, leaving any rainfall available to the tulips). Irises are best hand weeded, which is laborious, but, with a lot of patience it is possible to avoid weeding out a good number of the tulip seedlings.
One lesson to be learned from this is: If you want your tulip cultivars to last for many years in the garden plant them at least 15 cm deep (certain types, e.g. Lily Flowered, tend to be more perennial than others). If you are using them for bedding and are intending to throw them away after one year, there is no need to plant the bulbs more than 5 cm deep.